I've been promoting/pushing for GOG to release prepaid wallet cards in retail stores for well over a year now. One of the three reasons I haven't made the full switch to buying from GOG only is the fact that it's a lot harder for me to buy from GOG since I don't have a credit card and I don't feel comfortable using a debit card online.. and, unfortunately, you can't use prepaid credit cards since the companies behind those made some dumb "no transactions outside of the US" policy. Steam is the easiest way for me to buy games because they have Steam cards available in stores. If GOG had those, I would literally only buy from GOG unless a game wasn't available here.
zlaywal: You can use paysafecard
Unfortunately, paysafecard isn't super easy to find for everyone. I live in a small town and the nearest vendor for paysafecard is about a 30-minute drive from my home.. which means I could only really do that if I were making a trip out that way for more than one reason.