They kinda stuck their foot in their mouth on this one. If the time is set before gil is the globally accepted economy, it's economically -worthless- and thus no one would have any logical reason to accept it. Moreso, the currencies such as Bronzepieces and such are the widely accepted currency at the time and thus things should cost such...however, since those are the widely accepted currency, you'd think there'd be some way to aquire them somewhat easily.
This leaves you with either the gaping plothole of being unable to acquire the common currency at the time, or the gaping plothole of them using a currency that isn't even accepted yet. Frankly I'm hoping that maybe they use perhaps a different-but-same version of the currency that perhaps can't be taken back to the future or simply is different in such a way that it can't be used to upgrade relic.
Does people even read up on FFXI history or at least follow the storyline? <.<
This is the third time I say this... gil was already used at that time, mainly in Jeuno and by traders. We enter the war around at the time the Archduke of Jeuno unite the three nations to fight the war together. The reason they used three different currencies is because they were partially at war with each other as well. The Archduke, however, realized that unless they decided to help each other out, it would result in everyone losing the war. Jeuno was a neutral ground, so the best place to hold the emergency meeting was there. That's also why Jeuno drop all three currencies, due to being neutral.
And since the three nations decided to help each other out, it would, naturally, be wise of them to use the same currency, thus they start using gil. However, they don't officially release Gil as the official currency at this time, since it would naturally be confusing for everyone to change the currency to gil in a state of emergency. That's why they officially change the currency to Gil
after the war is over, to celebrate that they, together, won the war, and as an act of trust between each other. To celebrate new, peaceful times, together, as allies.
That's also why they were so reluctant to introduce Ballista, out of fear that it could start a new war between the nations.