One of the easiest and fastest ways to make gil (kinda surprised no one has mentioned it yet) is to level a tank job to 60 and just run leveling roulettes for cracked clusters for Grade 6 materia and sell them, it's basically 90,000+ gil per dungeon. Outside of that another easy way is to turn in gear you don't need into seals via delivery missions then buy glamour prisms with your seals and sell them, I make 1 million+ a day just from selling glamour prisms alone. I'm kinda surprised no one has mentioned either yet.