Credit cards are not common in Germany? What?!
Anyway - you don't need a credit card. You can pay through paypal or through paysafecard (paysafecard can be charged in a similar way to steam or google play - with cards that are bought in a store. They're available in Germany. You can find store near you here)
Credit cards which are perfectly common here in Czech Republic are working perfectly with GoG. But who knows. Maybe they havnt Visa in Germany.