Don't forget ease of use. Gold don't rust, don't rot, and is easy to represent large value with small weight.
c.f. Lead. Lead has been valuable in its own right, and even used as currency insofar as bars with an intrinsic value related only to potential use, or similar. Gold is pretty and rare and difficult[1] to fake. It's 'ease of use' would otherwise only lend itself to artwork crafting (you wouldn't make a solid gold broadsword, except for vanity's sake) and the non-rusting/rotting property merely makes it a constant. In actual value to a person, a gold coin would only be worth a large value because of its representation of a large value that other people would accept. Flooding the society with an exces of gold coins would not make everyone rich, just leave those already possessing of gold coins with a lower 'purchasing power' than they previously had. And as soon as you find that your ten gold coins can no longer buy you the loaf of bread, and you wish you still had the sack of grain you'd received them for, earlier.
What's the Star Trek currency[2]? Gold-pressed Latinum or something. Because gold can be synthesised easily through Replicator technology, but (in a hand-wavium fudge of technobabble similar to the dilithium-looks-like-quartz-but-is-4-dimensional-so-can-contain-antimatter-reactions thing) apparently Latinum (or whatever it is) cannot be, for whatever reason. So 'chits' of GPL are standard that is accepted for inter-and intra-species trading throughout the 'civilised galaxy', or at least parts of it that directly abutt the federation and end up featuring in Enterprise[X]- (and DS9-)based storylines.
[1] Across most tech-levels of society, given that any particular measure to counterfeit has been pursued with appropriately sophisticated measures to detect such counterfeits. Though usually all that's needed is familiarity with The Real Thing to readily identify attempts to pass gold-plated/painted lead, iron pyrytes, etc, with the occasional "Euraka!" moment adding displacement tests to the fold.
[2] At least outside of the "credits" economy within the federation that is largely glossed over, but I find surpsrisingly 'communist' and "to each according to their need, from each according to their ability" given that it was a 60s/70s US series in classic "better dead than red" era.