I tend to be a 'broke' player (I only have 10-50mil most of the time, and that's as an omnicrafter), but once you hit cap, I've found a few different ways to at least keep a slight positive...an ultra-lazy way to make gil, if you will:

1.) Do whatever dungeons/trials you were going to do. Roll 'greed' on all the gear you don't need for upgrades. (Rolling 'need' if you don't need it is rude, and not recommended.)
2.) Prior to patch 5.1, if you haven't already, level desynthesis and break those items down. If you're unlucky, you might get a mat worth only 500-1000 gil. If you're moderately unlucky, you might get clear demimateria (vendors for 5k gil). What you're really aiming for here, though, is Battlecraft Materia III. This regularly goes as high as 12.5k each week, sells pretty quickly and regularly, and you can get more than one per piece you break down.

Using these two alone, I can easily make 50-100k in a day without even so much as trying. However, this might change in 5.1 when desynth no longer has levels and anybody can do it.

3.) Gather maps. You can get one every day if you have a lv75+ gatherer that sells for roughly another 100k, and sells pretty quickly.
4.) Turn in goetia for mats to sell to high level crafters. I don't know the exact exchange rate offhand (I neither buy nor sell in that market [I'm lazy, remember]), but I did see just last night somebody offering a million gill for 2k goetia worth of mats. That would imply it's worth at least that, if not more.
5.) Level and send out your retainers. Quick exploration in particular is my favorite choice here, as while it's not always consistent, it can give you items that go for hundreds of thousands of gil, or even millions in rare scenarios. To fun ventures for your retainers, take the items you can't desynth, turn them in to your GC for seals, and use the seals to buy more ventures.

At this point you should have a decent baseline economy going. It won't make you rich (roughly 1-2mil/week), but it will fund any gil needs you have for almost everything the game has to offer.

6.) Custom Deliveries. Buy mats/materia, and sell them. This should add another decent chunk of change to your wallet.
7.) Wondrous Tails. The rewards can be sold.
8.) Sell those extra materia you don't need (particularly grade 6/7/8)
9.) if you can get enough gil for grade 6 materia, convert poetics into materia. If not, convert poetics into demicrystals and vendor for roughly 15k gil/800 poetics.

This should be enough while being lazy, but now you can start trying to build gil (for large purchases, for example)

10.) Level crafters to at least 10 levels below cap. This is practically free between GC turnins and beast tribes, and allows you to repair your own gear basically for free (and on the fly, to boot!). A gil saved is a gil earned.
11.) Level crafters to cap. Now you can meld your own stuff, too.
12.) Do some research into how to 100% HQ crafted recipes. HQ stuff and sell it. However, be wary of crystal costs, etc.
13.) Learn to play the MB.

Once you've done these, you'll probably be far richer than me (I'm broke, after all). Good luck with your gil. However, bad news: it's pretty much worthless after the first 50mil unless you're going hardcore into glamour/housing.