According to the Final Fantasy Wikia,

Gil (ギル, Giru?), also translated as GP, Gold, and G, is the currency in all of the Final Fantasy games. It is acquired throughout the games and used for making various purchases and occasionally for some abilities such as Gil Toss.

We don’t know what gil looks like, but we can imagine:

In Final Fantasy X, gil coins come in several colors (and presumably metals), and each bears an image on one side and the value of the coin on the other. In Final Fantasy VI and Final Fantasy VII, gil appears as silver coins with a hole through the middle, while in Final Fantasy V gil coins are gold with a hole. In Final Fantasy XII, Penelocan briefly be seen holding a pair of coins presumed to be gil, one silver and one gold, with unclear detailed markings on them.

Also,

This suggests that gil is loosely based on the Japanese yen (円) or ancient Chinese coins, with the Final Fantasy X gil coins appearing very similar to yen coins. The yen, along with other Chinese and Japanese currencies, such as the mon, have holes in some of the coins, a trait dating back to feudal times when coins were carried on string.

But this isn’t the only reason. We can see that nearly every gil you receive is in denominations of one and there aren’t smaller denominations. This would essentially be the equivalent of a penny. Every monster which drops 100 gil has on his person a loonie, a dollar or a pound.

But consider the first Final Fantasy. If gil is the equivalent of a penny or pence, then that means a knife would be roughly ten cents. But that doesn’t sound right, does it?

Of course, we also might have to justify inflation. Using this site, we see that in 1913, 10 cents has roughly the same purchasing power in 2016 as 2.42. That’s the cost of a pocket knife. And that’s just going back as far as 1913. And don’t forget, it seems like monsters contain the gil. So if you want money, you have to barter fur and meat for gil.

Final Fantasy VII’s setting was originally New York City, right? Using this infographic, let’s look at Aerith house, as it would be using American currency. The value estimates at $1,886,304, 37,726,080 gil, or 215436519.86¥. By comparison, the beach house at Costa del Sol is 300,000 gil, $6,000,000, 685265534.68¥.

And all of this assumes Final Fantasy worlds have a currency system like ours.

Fun fact: According to Final Fantasy IV, the gil is named after the Gilbart family of Damcyan. That must mean that Damcyan could be the originator of the currency and the richest family in Final Fantasy history…