From the official Strategy Guide ...
Leveling Spot # 3:
Eden - Leviathan Plaza
Spot Details:
The Adamantoise offers some of the best rewards in the entire game: 40,000 CP, a Platinum Ingot as its normal drop (sell price 150,000 Gil), and a Trapezohedron as its rare drop. The latter component has a resale price of only 10,000 Gil, but is essential to create "ultimate" weapons and costs a massive 2,000,000 to buy.
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In addition to "premium" components that are designed purely to be sold, there are several standard components with a Gil value that far eclipses EXP or multiplier increases that may offer during weapon or accesory upgrades. Acquiring these can be a great way to generate funds for purchases.
Name Sale Price (Gil)
Gloomstalk 1,000
Sunpetal 1,000
Moonblossom Seed 6,000
Starblossom Seed 13,000
Succulent Fruit 1,750
Malodorous Fruit 4,000
Green Needle 3,500
Perfume 12,500
There's a gil farming guide at gamefaqs, but it deals with near endgame, endgame, and postgame farming. In the beginning of the game there's not really much you can do to farm gil other than selling components. Most components sell for a pittance relative to the cost of anything you might actually want to buy, so it's generally more worth it to use them to upgrade your accessories (heavily upgraded +str or +magic accessories can really make the early game easy.) There's also not much to buy anyway. PSICOM or Guardian Corps enemies have a chance to drop a Credit Chip, which sells for 500 gil, and has no other purpose. There's a few encounters in the early game that I think always drop them since they're sort of like minibosses. You won't see very many enemies that drop them until around chapter or 8 or 9, when you get to Palumpolum.
Here's where you can start farming in earnest. Spoilers follow. Each area is better farming than the last.
When sneaking in to Palumpolum there's several groups of PSICOM/Guardian Corps dudes patrolling that you can farm semi-effectively. You can go from the save point, to the end of the tunnel, then come back. Only the earliest ones will have respawned, but you can save and reload to respawn them all.
A little bit later after after Snow meets back up with them, is an area where's there's more larger groups of soldiers. These also are the first enemies who can drop Incentive Chips which sell for for 2500 and like the Credit Chips are meant to be sold. There's usually one or two in each group who can drop it and have Credit Chips as their common drop. Once you get the rhythm of 5 starring the fights (I found Rav/Rav and Rav/Com paradigms, with Med/Sen or Med/Com only when you're really hurting to be the only feasible way) you'll normally get maybe 3-4 credit chips and an incentive chip or two per run. There's also a couple enemies that can drop Rhodocrosite which is used as a catalyst. It takes about 5-10 minutes per run, and there's a save point on either end of the area so you can just save and reload.
The next area is at Hope's dad's apartment, where there's two groups of PSICOM guys that endlessly respawn if you walk a short distance away. The apartment is roughly circular so you can just run down the hallway in one direction until you get tired of farming them. There's more incentive chip droppers in these groups. It's also about when there's some nice accessories you may want to buy (I think 2nd tier str and magic stuff) and upgrade. This place is pretty awesome since it's essentially infinite enemy density. About 5 seconds between fights.
The last is around the middle of Chapter 10 (I think) where you're on the big airship, and there's a big section of hallway with about 7 groups of 3-5 PSICOM guys that drop both Credit Chips and Incentive Chips. There's a savepoint as well as a door to another section of deck you can go through to force respawns. Basically same song and dance as the previous parts.
After that the next (also much more effective) is the chocobo farming which is in linked guide.
Best Answer
The best way to farm gil is Sacrifice farming in Chapter 13, but you can't do this post game so you should farm all the gil you need before you beat the game.
If you are using death to kill adamantoises then the most efficient way is to perform this loop:
Get 5tp, save game, fight toise, summon, spam death.
If you die, retry before the stomp will kill you, if death succeed check the battle results. If you got nothing, reload the save game and start again. If you got what you wanted go farm 5tp and save the game again and start over.
Farming TP is fastest if you fight the gorgonopsids and goblins with the equipment which boosts TP gains.
This farming method is pointless and slow in my opinion. You can't get Death until after you defeat Barthandelous in Oerba. It is much more efficient to just beat chapter 12 and do all your farming in Chapter 13. Chapter 13 has the most efficient gil farming and most efficient CP farming in the same area. The only reason to farm 'toises is for Traps. And once you have max stats you won't have to do slow summon-death strats you can just beat them directly. Farming traps is even faster against long guis.
Delaying chapter 12 to go backward to the steppe and fight toise with bad strats isn't a good idea. If you have to use death to fight toises you are too weak and its a waste of time.