On Gil:
If Final Fantasy XI is anything to go by, 'gil' is just coins of various precious metals; in FFXI, gil appears as gold and silver coins.
Oddly, FFXI is one of the most fantastic (in the literal sense) of the recent games, but its currencies are rather mundane; even ancient currencies worth millions of gil are just lost mints of coins and, most surprisingly, paper bills, such as Bynes.
The only odd currencies are the beast races' monies, which have odd shapes and appearances and are minted separately by the different beast races.
Gil probably isn't the same for every Final Fantasy, though. It was said that gil was a mistranslation of gold, and while this may be the case, at least initially, there's a clever rationalization for it: 'Gil' is said to be a contraction of Gilbart, a common name in the powerful Damcyan dynasty in Final Fantasy IV. The name was catchy though, and it stuck. Since then, gil has taken on its own sort of identity spanning the mythos.
For example, in the early FF's like I through IV, which are very 'old world,' it's probably all coins, precious metals and whatnot. In V, VI, IX and XII, the settings are a little bit more advanced and probably includes bills and certificates in the more advanced cities like Karnak in V, most of the cities in VI, Lindblum in IX, and Archadia, Rozarria, and perhaps Rabanastre and Bujerba in XII.
The worlds of VII and VIII were set roughly parallel to their release dates, so the currency in the world is probably not any different than it currently is in America, Japan, Europe, Australia and other countries these games were marketed in. Esthar probably had some weird currency. Possibly all done electronically, like the pipedream people think we're approaching someday.
I can't even imagine what they use in X. Spira is a total cipher; a setting with absolutely no setting details given about it other than a fucked up Vatican stand-in, a love of underwater soccer, and an almost indubitably Freudian love of spheres. It's probably just coins, honestly. You could make a case that they use spheres, but they do have coins, and it seem like if they have coins made of precious metals they'd use them for currency, or at least would have in the past.
Spheres probably aren't used as currency; if they were, it seems like they'd call their currency something other than gil, like gil spheres (DUUUR) or some other catchy but less obvious name. They'd also need a reliable but not particularly... accessible method of harvesting or creating new currency spheres, which doesn't seem to be the case. Spheres themselves are another cipher in FFX. We see a single instance of where spheres come from, but it's just some bluish pool with a crystal jutting out of it on Mt. Nibel. It just sits by the roadside and apparently spheres pop out of it every now and then that people somehow know how to use.
No other instance is presented of how spheres make their way into common use, and I assume without some method of harnessing or some way industrializing the use of spheres as currency rather than as a good, I'd have to say using them as such is unlikely and would count as bartering rather than buying.
What I can say with relative certainty about currency in Spira is that they probably don't use paper money like bills or certificates issued by a bank or government authority. With the exception of Bevelle, any place on the face of Spira is liable to be wiped out by Sin, meaning the only safe place to hold a supply of precious metal to back currency is Bevelle. Otherwise, any branch bank holding precious metals could be wiped off the face of the earth at any time, rendering paper money unstable and dangerous. Travel is dangerous and time-consuming in Spira, meaning if Bevelle were the only place to redeem money for gold or silver, you're out of luck if you live... Well, really anywhere outside of walking distance. What if you had to walk to your state capital to do any banking?
There's a slim chance that Yevon could have issued fiat money, but fiat money is invariably preceded by money backed by precious metals, and gives the money value based on the stability of the state itself for giving up stability tied to the metals by which it would otherwise be backed. Meaning there probably wouldn't be a concept in place for fiat money to arise from in the first place, and that if it did exist and the world ran on fiat money, the world economy was certainly destroyed following certain apocalyptic cataclysms I won't bother to recount. Thanks, Yuna! The last remaining Ronso must have laughed their asses off, since they almost certainly don't use money at all among themselves.
In closing, it's probably worth acknowledging that gil is likely an abstraction. No way does every nation in every world of every Final Fantasy bow down to a single decentralized currency. The real world has currencies that act like I think gil does, though, such as the Special Drawing Right or SDR, which, in spite of its terrible name, has a value amalgamated from several of the world's foremost currencies, giving it a very stable and relatively high value that has made it very attractive to investors in recent years with the global economic downturn. If they're given the chance to make anymore SDR's, you should probably invest in some, as unlikely as it is you'd be able to call dibs on any of them.
Gil probably functions similarly as a game mechanic; anytime you get a big wad of cash from killing a pack of wolves (?????), or some girl tells you a flower is only a gil, the game is just telling you how much it costs in gil, the game's convenient measuring stick, rather than constantly using local currencies and making you how many shebogis your fritzkins are worth everytime you take your airship from Kalm to Costa del Sol to pick up a crate of their world-famous Soft.
Going back to FFXI's example, it's possible gil do exist in the games, at least in some worlds; as I said, all nations had their own currency in FFXI prior to the rise of Jeuno as the economic superpower. It's entirely possible that all these currencies still exist and are used in their own countries, but all merchants also accept the strong and stable gil, which is backed by the thriving and intractable Jeunoan government, making it attractive to adventurers, who travel the world at a moment's notice and can't be bothered to exchange their Ranperre Goldpieces for Bynes.
Conclusion: In my opinion as a nationally-recognized Cryptonumismatist, Gil is an utterly mundane and boring currency and I can't imagine why anyone has seen fit to include it in this thread.