and that some GCs don't work... yet
The GCs that he tried didn't work. There weren't any that did. What makes you think that will change?
The same way certain GC's work with other services like this. You need to try ever BIN possible. Believe it or not he actually holds back some information.
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Looks like his account was flagged when he tried to load his second GC
It's the first few digits (I believe 6) of the credit card, tells you who the issuer of the card is
Correct and some issuers have many different BIN's for GC so you have to try them all. Disclaimer: I do not use Revolut.
Any way to get the app with a American phone number?