Fun fact, There are actually both gold and bitcoin backed credit/debit cards.
The merchant sees his favourite currency, the customer sees his favourite currency, and all the exchanging goes on behind-the-scenes. I don't see why a bitcoin or gold debit card is materially different to any other foreign debit card, from the perspective of either the merchant or the customer.
We are definitely not there if we ever will be (personally I am skeptical most of these tokens will turn out to be more than fads few years from now). Merchant still gets paid with USD, GBP, JPY, EUR or whatever currency is local to the merchant.
It also must be more inefficient to use Bitcoin or gold card. Because there is an additional layer of conversion from BTC/gold to fiat which I believe is not as advantageous as paying with fiat which uses daily Visa/MC interchange rate (Visa/MC set exchange rate for currency pairs once per day so it doesn't fluctuate when you buy 2 coffees per day for example) and is as good as you can get.
I imagine whatever service you use to change BTC/gold to fiat in the background must incur some fees albeit small ones (Coinbase or something else that's used by these cards, the API call you make to Coinbase to change BTC to USD will charge you small fee). So you will be paying more on fees / get worse exchange rate.
I still don't understand what's the advantage of using these Bitcoin or gold debit cards as it seems you are just paying extra fees for extra layer of conversion. I'd stick with old school boring bank debit cards since transactions are done in fiat anyways.