I can't make any meaningful statements on the legal situation of
Carmageddon: Max Damage but if you can't buy it directly, you probably won't be able to redeem a gift code either. I would assume that this is less a matter of
GOG not wanting to take your money but rather a matter of German law doesn't want underage(?) Germans to have this game. I'm not suggesting anything about your age. Judging from various wishlist discussions[¹][²], there
could be ways to verify your age to GOG but GOG has yet to implement it. You could search for those wishes and vote for them in the hopes that it will make a difference.
Also asking for a gift seems kind of rude. I mean, come on: You should at least offer something in return.
[¹]
Optional age verification (e.g. for German users) [²]
Add SOFORT Ident and Post Ident to verify age [³] There are most likely more. Those are just the ones I can remember from the top of my head and which might be relevant to you.