Earning Gil Quickly
There are two methods you can use to earn gil quickly—the legitimate way or through the use of cheats.
Legitimate Methods
The easiest and most efficient method to earn gil is to sell Mastered Materia. Mastered Materia is Materia that you have managed to level up to the maximum allowable level. The best Materia to sell for this purpose is a Mastered 
All Materia.
You will find several All Materia throughout the game, which makes it relatively easy to sell one or two once you fully level them. A new Materia is born once you fully level one of them up, which means that you can move on to level up the next Materia as soon as you’ve mastered one. You can keep a rotation going so that you don’t waste AP.
A Mastered All Materia sells for 1,400,000 gil.
Maximum Materia
Warning
Note that there is a limit on the amount of Materia that you can have in your inventory. Few players will run into this limit, as you can hold up to 200 Materia at any given time (which excludes Materia equipped to characters). However, for Completionists, or those stocking up for experimentation purposes, etc., you should be aware of this unlisted limit.
Once you reach this limit, the new Materia that you receive will randomly replace the Materia that you have in your inventory.
Cheats
The easiest way to earn gil if you are comfortable using cheats is to utilize the W-Item Materia item duplication trick. Check out the Cheats section for more information. You can then sell the duplicated items.