where do you get the most amount of gil and quickly? in which area?

Omega Ruins. Look for battles with a treasure chest in them. Open it and if you pop a Mimic (and can defeat it) you'll be rolling in the gil. Especially if you have Gillionaire on Rikku :)

Alternativly you could do the quest at the Moster Arena in the Calm Lands then fight them. They net good items that you can sell for a reasonable profit. of course, thats assuming you have buffed up characters