"it changes, but it changes for everyone at the same time"

No, it doesnt.
It not only changes due to the natural change in gold spot, but also by the relative change of exchange rates between each currency and the dollar (or whatever new currency you wish to value the gold in).
It would only be the same for everyone if inflation (and hence depo/repo rates) were identical across the globe.

Anyway, there will be a global currency sooner or later, the Yuan