Randalator: It's not GOG or their current payment processor per se, 99% of the time it's the bank that blocks international purchases in general or some automatic fraud protection randomly wetting its pants.
The card in question works fine with both domestic and international purposes in my case. It's just on GOG that I'm having issues, so yeah... will wait for a week or so and hopefully it will sort itself out. It's not the first time I've had this happen, but it's been years since I've had these sort of issues.
I know it's not the bank, because I'd see the rejection in that case, so it looks like the payment processor is the one doing the rejecting or whatnot.
P.S.: Also tried one of those digital single-use cards, and it also got rejected :/.