Alright, several years ago I used Visa and MasterCard gift cards to buy a few games on GOG. I had to use a guest checkout thing on PayPal because the "normal" way didn't work. At some point, the guest checkout ceased to exist in the form I used and now it doesn't work at all. Eventually I gave up and quit even trying to buy stuff from GOG. I have waited a few years before trying again and I come back to try it because its DRM free but gift cards still do not work at all. This is ridiculous. Steam works just fine with gift cards. Why would an online store not accept the prepaid cards? Should I blame GOG or Visa/MasterCard for this?

Are they standard prepaid debit visa/mastercards that are confirmed recently to work online buying things in general?.
I havent used any in the last 12months but they worked previously for me.

They are OneVanilla prepaid cards. I just bought something on steam with them tonight. I am able to buy things on amazon with them.

On GOG's support page they state that a lot of prepaid Visa cards do not work for payment outside the USA. Could this be the issue? GOG is Polish after all.

Well, which ones do work?

I have never set my foot on the American continent(s). I have no idea.