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.