I don't know what it going on, but for the last month, I have been unable to buy anything from GOG using my VISA card. I've even tried using PayPal, and even that refuses to process the transaction.

Note that I have bought games from GOG before using my card.

Have you already checked it over with your bank to ensure it's not an issue there?

Maybe you made some small transactions that made them freeze the card, happened to me a few times. Need to call your bank

Are you sure you didn't just forget to renew your card?