Gold is almost entirely propped up by central banks who control the majority of the world's supply. They never had to pay for that gold themselves, since it was obtained via the taxing powers of the respective governments.

How vaulable would gold be if all the central banks auctioned off their supplies to the highest bidder and then refused to buy any more?

In terms of ensuring Bitcoin's survival in the face of hostile and well-resourced interests, I think a better model to look at is the Tor project. Why are they able to survive in spite of a lot of powerful interests who'd like to see otherwise?