Without spaces. Remember to put in the expiry date and CV2 as well. Oh, and put your name exactly as it's written on the card! See the example I just made: Pic

If that still doesn't work, you can use paypal without making an account. You just pick paypal and go to the page where it asks you to sign in. There's a little link there saying "Don't have a PayPal account?
No problem, continue checkout".

Now it worked, thanks guys!

Apparently my card was not allowed to work in teh interwebz, I needed to activate it via my bank. Nice thing about it was that it was never mentioned anywhere, I fluked out and found it in their online bank site :)

I got a Visa Electron debit card. No problem paying with it. Actually I'm forced to pay with it cause I also have a Mastercard with which I used to pay but now my bank implemented 3DSecure (I think it's called) and GOG does not support it. Both cards are tied to the same bank account. Weird they only implemented for one but I'm not complaining :)