Gongaga is located on the southernmost section of the western continent. The location is represented by a small burnt-out reactor on the world map that can be hard to see as you pass it.
The story of the game will actually take you past Gongaga once you leave the Corel Prison with the Buggy and are on your way to Cosmo Canyon. Check out the Gongaga walkthrough page for more information.
This area is actually considered an optional area to visit, and while there is some additional story information and some dialogue, as well as some useful items, you can bypass this area and drive past in your Buggy if preferred.
There are some useful items that you can pickup in this area though including:
Titan Materia
Deathblow Materia
White M-phone
X-Potion
There are also a number of shops in this area including:
The map below shows the location of the Gongaga on the World Map:
Hardedge
Grand Glove
Atomic Scissors
Striking Staff
Diamond Pin
Boomerang
Headband
Mystify Materia