There really isn't a way to get "quick gil." The easiest way is to farm high-volume items, like beehive chips or other crafting materials. You can also try your hand at a BCNM. A popular choice is the level 40 capped "Under Observation," as you have a chance of getting Utsusemi: Ni or a Peacock Charm. (around 300k and 2mil, respectively) Royal Jelly is also a nice alternative to Under Observation, as you can get Utsusemi: Ni and an Archer's Ring, the latter of which goes for near 500k on my server.