sadlyrematch: I've also tried like 4 other cards and each one results in a decline saying it's an issue with my bank (it's not).
vv221: This is most probably a false-positive problem from GOG anti-fraud system, especially if you tend to use multiple card numbers from a same account.
Good thing you opened a ticket with GOG support, as this is probably the only way to unlock the ability to pay with your account. I got a similar problem a while ago, and it stayed like this a whole year until I managed to get a hold of GOG support and convinced them the problem was
not on my banks (plural, I tried paying from multiple bank accounts) side.
Prior to my main card being declined repeatedly I never used other cards on the site, so I doubt it. It's an issue with GoG's credit card processor implementation. The same main card was also declined last night, but I retried changing literally nothing, and it went through fine, so I figured it was just a tiny bug in their site. Seems to be a much bigger bug now.
There seems to be people on Reddit experiencing the same problem over the past few days, in some cases with cards they've used on GoG before.