Being a good store of value, is itself - a value. It's not entirely arbitrary because you need quite a lot of characteristics to make it fit. It needs to be relatively widespread to encourage widespread acceptance of its value (rather than just granting Goldland a defacto king-maker style monopoly on wealth, which nobody beside allies of Goldland would recognize), relatively scarce so you're not requiring hundreds of tons of it to cover even a moderate national level transaction, it needs to be safe to handle, stable, solid at any practical temperature, and somehow relatively easy to verify the authenticity of. I can't really think any other element that fits the bill as nicely as gold.

Put another way, if we look at all the parallel realities of Earth, even prior to the discovery of gold, I suspect the overwhelming majority of them would end up using it as a major store of value. It's just really perfect for it! We may even find alien civilizations, assuming a comparable elemental distribution to Earth, doing the same!