Gold supply is still unknown and it's available till the last gold mine is not shut down or there are chances that you can find another one year after also and it has been in circulation for ages but on the other side btc supply is fixed and capped to 21million only and that also till 2140 with POW mechanism and you can't change it without concensus.

And it goes same with the gold also as they have not started mining at other planets but suppose the human found life on another planet then you would need internet and phone to use bitcoin so it's non physical indentity mande it usable everywhere.