The Mt. Gagazet portion of the game takes place after the team has traveled through the Calm Lands.
There are a few more cutscenes once you work your way up the pathway towards Mt. Gagazet. The first cutscene involves Lord Kelk Ronso and the other Ronso of Mt. Gagazet attempting to prevent the traitorous team from entering the mountain.
Once the scene has concluded you will have access to the Save Sphere. Save your game before you continue.
One last reminder… There is an upcoming fight involving just Kimahri in the next area of Mt. Gagazet. This fight can be particularly difficult if you have not been using Kimahri as a core member of your team and if you have not been leveling up his Sphere Grid.
You may want to take this time to backtrack and fight some random battles with Kimahri, either in the Cavern of the Stolen Fayth or back in the Calm Lands.
You should also make sure that Kimahri has learned either the 
Steal command or the 
Mug ability before you continue. You can do this by either using a 
Special Sphere to teach him the Steal command or you can use a Skill Sphere to teach him the Mug command (assuming that Rikku has already learned this ability).
You can steal up to eight 
Lv. 3 Key Spheres from Biran and Yenke depending on how lucky you get. This is one of the very few places that you can obtain the rare Spheres so it is highly recommended that you have Kimahri learn to Steal or Mug.
In Game Description of Biran Ronso: Blocks attacks on Yenke with Guard, except when separated after performing a Bulldoze. Casts Mighty Guard just once after losing HP. Kimahri can learn many techniques.
In Game Description of Yenke Ronso: Blocks attacks on Biran with Guard, except when separated after performing a Bulldoze. Casts White Wind just once after losing HP. Kimahri can learn many techniques.
HP: Varies based on level
Biran and Yenke both have a number of Lancet abilities that Kimahri can learn from both of them:
Biran: Thrust Kick, Self-Destruct, Doom and Mighty Guard (after he has used it once)
Yenke: Aqua Breath, Stone Breath, Fire Breath and White Wind (after he has used it once)
Use Kimahri’s Lancet ability to learn any of the abilities that you do not currently have. Check out the Overdrive section for more information on Lancet and Kimahri’s Overdrive ability, Ronso Rage.
All of the regular attacks that you use against either Biran or Yenke will be ineffective as they both permanently use the Guard command on one another which causes regular attacks to hit for 1/4 of the regular damage. Use Lancet and Ronso Rage after you have picked up all of the abilities above to slowly bring them down. The two Ronso will no longer use Guard on one another if they are on opposite sides of Kimahri (after either of them uses the Bulldoze ability).
Make sure that you take the opportunity to use the Steal command or Mug ability (if you had Kimahri learn either of them prior to the fight) to steal as many Lv. 3 Key Spheres as you can.
If you defeat Biran first, Yenke will remain under the Haste status effect for the remainder of the fight. If you defeat Yenke first, Biran will remain under the Berserker status effect. While he is under the Berserk status effect, you will not be able to learn any new abilities from Biran, so make sure that you have picked them all up first before you kill Yenke (if you kill him first).
Steal:
Lv. 3 Key Sphere
Drops:
Return Sphere (common)
Friend Sphere (rare)
Return to the Save Sphere in the previous area and save your game.
Continue down the pathway for another cutscene. This one involves the Ronso tribe sending off Yuna and her guardians.
There is a treasure chest just off to the right of the start of the mountain trail. It contains 20,000 gil.
There is a small plateau off on the left side with a treasure chest that contains two 
Mega-Potions.
You may run into an enemy called a Bashura in this area (shown in the screenshot below). They can be particularly troublesome once they get their defenses up and use “Psych Up” as they will start countering all of your attacks.
They have a lot of HP (17,000) so a good strategy to use against them is to have a character cast 
Bio (Lulu or someone else if they have also learned it). Each ‘tick’ of Bio (each time they have a turn in battle) it will hit them for over 4,000 damage.
Now its time to start making your way down the path. Lulu will tell you about the grave markings as you pass by the first one. Each of them marks the way for guardians who have failed in their pilgrimage. You will have to battle a few fiends after each grave marker as well.
The pathway through Mt. Gagazet is fairly linear.
There is a trail that goes off to the north part way through the pathway. Following it up northward will lead you to a Braska’s Sphere (similar to a 
Jecht Sphere). The Sphere contains a flashback sequence involving Jecht, Auron and Lord Braska.
Finding all of these Spheres is part of the Jecht Spheres side quest and is also part of the requirements to obtain Auron’s new Overdrive abilities.
This will be the last of the Jecht Spheres for you to obtain if you traveled back to Besaid Island after defeating Spherimorph in the Macalania Woods. If you haven’t returned back to Besaid Island yet to pick up the Jecht Spheres you may wish to circle back and revisit the side quest page (using the link above) to go and get them now.
There is another short cutscene involving Lulu and the group at the second grave marker. She will explain that the summoners who die on the mountains are not sent to the Farplanes. Many of the fiends in the area are likely summoners who failed in their mission.
Go westward once you reach the next Y-intersection (shown in the screenshot below). There is a treasure chest at the end of the trail that contains a 
Defending Bracer for Auron.
There is a man named Wantz that you can speak to along the way. Wantz is O’aka XXIII’s brother. Speak to him to purchase any items that you need. He sells a very strong weapon for Lulu called 
Booster Cactuar (abilities include 
Magic Booster, Magic +10% and Magic +5%) and a great weapon for Rikku called 
Survivor (Alchemy and Strength +10% with two additional customizable ability slots).
It likely hasn’t impacted you up until this point of the game, but there is actually a limit to the number of weapons and armor that you can carry during your travels through Spira. In order to free up space, you should consider selling any of the weaker or duplicate weapons and armor that you have picked up.
If you are a completionist you may be worried about parting ways with irreplaceable items, but don’t worry. Weapons and armor in Final Fantasy X are not unique and can be replaced. In fact, with the exception of treasure chests and boss battles, most of the weapons and armor you acquire is by random chance. Sell some of your items before you venture past Wantz.
Wantz will appear later on in the game in Macalania Woods but he will only appear if you speak to him when you pass through this area. Wantz’ second shop includes the 4 empty slot weapons and armor such as 
Tetra Shield, 
Tetra Ring, etc. which are very important items for the end-game content.
Make sure you speak to Wantz on your way through Mt. Gagazet!
You will also want to purchase a fair bit of 
Holy Water. You can use 30 of them to add the Zombie Ward ability to your armor through customization and you will need this ability for an upcoming boss fight.
Take a left down another secret pathway when you approach the area shown in the screenshot below.
There are two treasure chests down below, one of which contains a Lv. 4 Key Sphere while the other contains an 
HP Sphere.
There is another grave stone up ahead. Lulu doesn’t have much to say about this one if you inspect it. There is a pathway that juts out westward but there is nothing at the end of that pathway, so ignore it and proceed southwest.
There is a Save Sphere at the end of the pathway which you can use to save your game.
There is a boss fight coming up in the next area which you should do some preparation for. Make sure that you purchased some Holy Water from Wantz (O’aka XXIII’s shop) and, if possible, that you have added the Zombie Ward ability to some of your armor and equipped it.
You will also want to have enough 
Phoenix Downs to last the battle (30 to be on the safe side). If you have been having difficulty (due to levels or due to play style) on any of the previous bosses you may also want to charge up all of your characters Overdrives as the upcoming boss is often described as the most difficult boss in the game.
Make sure that Yuna has learned Dispel magic as it will come in very handy during this next boss battle.
Continue forward to initiate the fight.
In Game Description: Predict the Mortiorchis’s actions by watching Seymour’s attacks. His Lance of Atrophy causes zombie, and Mortiorchis follows with Full-Life. Together they unleash Total Annihilation.
HP: 70,000
It’s another showdown with Maester Seymour! Kimahri and Yuna will be able to use Trigger Commands during this fight. Kimahri will gain +10 strength while Yuna will gain +10 Magic Defense.
Seymour will summon Mortiorchis which is similar to his side-kick during your last battle with him at Bevelle.
He will also use an ability called “Lance of Atrophy” which could potentially inflict the Zombie status on one of your characters. Equipping armor with the Zombie Ward ability as mentioned above will help to prevent this.
You should immediately use Holy Water, Remedy or cast 
Esuna on a character inflicted with the Zombie status as Seymour Flux will cast Full-Life on a character with Zombie which will instantly kill them.
Seymour Flux will banish Aeons just like he did previously so you should only use them as a last resort or as a last ditch effort if you are struggling with the fight. Have Lulu cast Bio on Seymour early on in the battle as it will add up to a very significant amount of overall damage and make sure to use Hastega on your party.
Yuna should cast Dispel on Seymour when he casts Reflect or Protect on himself so that you can continue to inflict reasonable amounts of damage. If you are fast enough to cast Dispel and remove his Reflect status he may even cast Flare directly on himself and will cause damage to himself.
The killer attack to watch out for is called “Total Annihilation”. It takes three turns to fully charge and it will likely wipe out your entire party.
Have Yuna cast Shell on your party members right before Total Annihilation is cast, or you can have Kimahri use Mighty Guard now that you have learned it from Biran, or lastly, have Rikku use one of her Mixes to cast Mighty G, Super Mighty G or Hyper Mighty G on your party. Use the Defend command right before the attack hits to reduce the amount of damage done by even more.
Other than that, hammer away with regular attacks and magics until you bring him down. Good luck!
Steal:
Elixir
Drops:
Lv. 4 Key Sphere
Tidus and Auron will reveal to Yuna that Sin is actually Tidus’ father, Jecht, and that they are on their way to defeat and kill Tidus’ old man. Auron explains that they will come to understand more when they reach Zanarkand.
Continue down the path once the fight has concluded. Go around the corner and, just before the Save Sphere, there is a treasure chest hidden between the pillars along the pathway.
The treasure chest contains the Saturn Crest which is used to create Kimahri’s Celestial Weapon, 
Spirit Lance. Check out the Celestial Weapons section for more information.
Save your game at the Save Sphere. There are more cutscenes once you proceed around the next corner that involve Tidus’ memories back in Zanarkand. Walk toward Tidus’ house when you regain control of him in Zanarkand.
Go across the bridge and in through the doorway as there is nowhere else that you can go. There is more dialogue once Tidus reaches the center of his old house. A Fayth that has been following Tidus around will finally greet him. Follow the Fayth up to the second floor balcony.
The Fayth will reveal that Tidus, Jecht and all of the people from Zanarkand are just dreams. The story continues after Tidus wakes back up.
Continue down the walkway going past the Fayth once the cutscenes have ended.
The next section of the game takes you through the inside of the Mt. Gagazet caves.
