Farming Gil?

If you're powerful enough you can tackle the large adamantoise roaming about in the main area on Grand Pulse. They drop an item called Gold Ignots worth 150,000 in any shop. That is the painful way about it and you will need high stats and high weapon levels to kill them.

Check TakeTakeShi55's videos over on YouTube for tactics. I've been working with him on the best way to kill those damn beasts and he posted a great video with our combined efforts (note that his stats are almost maxed out and he didnt use the summon trick):

www.youtube.com/watch?v=cVyru965A-8

Also you will need the "Best Chance" accessory to increase the Rare Item Drop rate for it's rare item which allows you to mold weapons into tier 3 weapons. The Gold Ignots is a regular drop item though. You can get the Good Chance accessory from the Moogle Shop to increase the rate of this drop item. Note that you can also mold/upgrade it to the Best Chance accessory.

Good Chance - increases regular drop item rate
Best Chance - increases rare drop item rates

I'm not sure if stacking them works, as I've tried using four of each at a time but the results seem to be same.
If you can't kill the giant turtles, then go for the Female PSICOM Comando Combat Guards (the blue ones with the night sticks).

They drop an item called Kousui or Cologne (could be perfume but my dictionary says cologne) that costs 12,500 Gill. It is a rare item for her that drops regularly if you have the Best Chance accessory attached. I was able to farm 23 of these within the amount of time it takes to kill a giant adamantoise.

Some math = 23 x 12,500 gill = 287,500 gill within 10 - 15 mins. Results may vary but you always get the item, and sometimes 2 or three depending on the amount you them you fight at one time.

When you get further in the game, you will also come across the Sacrifice monsters (possible the female guards converted into demons) who drop Kousui as a regular item, so use the Good Chance accessory to farm it off of them.
Currently, I have amassed 2,485,000 gill this way and I'm still going for more. Until I can kill the giant adamantoise faster, this seems reasonable to get my weapons ready for tier three upgrades.

Hope this helps as I had searched far and wide on the inter-webs for an alternative technique to killing the large turtles and getting gill. I have even visited a few Japanese sites and they all say kill the turtle. Killing them is frustrating and takes up too much time and even after you kill them, the drop rate for the gold ignot is very low (with the right accessories attached). That was just ridiculous in my opinion.

This was more rewarding and I was able to upgrade all my weapons to tier two, and have cash to purchase adamantites, dark matter, and *gasp* Trepahedrons (sp) for those tier three upgrades ... still to each his own.