To assume gil has a standardised form is naive.
Currency has multiple values to assist in exchange of fluctuating price of different product.
10p coin, 50p coin, £1 coin.
However, currency does not typically share the same name, value and shape if it is produced from different economic backgrounds.

Our coffers show gil. Not Limsa-gil, Ul-gil or Grid-gil.
I'd say it's a safe bet to assume the images above are seals.

We need to acknowledge that Final Fantasy as a franchise and fantasy RPG games in general have glaring issues with money.
-You killed a cow-
Obtained 10g!

To say 'something drops gil, that means their heritage is valid source' is nonsense.
If Garleans drop gil, we can't confirm they use gil because we can't trade anywhere with them.

The only way to confirm is through utilising this currency in an established trading facility or shop!

A more pressing question on my mind rather than 'Who is on the gil' is; which governing body do all city-states and alliances bow down to and reproduce their currency!
You can't ascertain who might be on the gil without knowing which economy it originated from!

Or they just came together and said:
"Yo let's put a God on the coin and shake hands"

My money is on Ul'Dah produces the coin, but the city-state leaders of old came together and agreed upon the value.

As for who is on the coin?
Female Hyur! :s