I'm curious about both during the game, and after completion. What's the quickest, most efficient way to do this?
You can make a lot of money farming the Cie'rex Cieth at Oerba 400.
With the Fragment-Ability that increases the spawn of rare monsters, you can find him rather easily, and you can kill him in < 10 minuts with the right strategy (which includes clearing his mob as fast as possible).
You get 100k gil per kill, plus up to 3 items (I don't know the english name for them, sorry), which sell for a total of ~50k.
Edit: I think he's called 'Raspatil' in the English version, sorry.
I would like to know the answer too. For me though, I gained a lot of gil from grinding enemies on repeat. I went to Academia -400- in front of a "red block" where you constantly get into random encounters and just mashed the X button while I watched a movie. Maybe not the most fun way to do it, but it worked. Before I beat the game I had 4 level 99 roles and about 200k gil. I'll let you know if I find a better way.