I had the game on my wishlist all the time, now i wanted to buy it and as i searched it i noticed that i can't find it on the store.
What's happening? D:

Change your PC's region code. It works with locked sites like Japanese only game sales on locked websites, as well region locked multiplayers in Asian MMOs. You may also change your PC's internal clock in order to mirror the situation perfectly. :)

That's odd, as the retail version was actually sold in Germany back in the day...

Unreal Tournament and UT 2004 were removed from the store for anyone accessing it with a german IP adress just this June: https://www.gog.com/forum/general_de/blockierte_spiele_fur_deutsche_ipadressen/page1
Disappointingly, GOG decided not to give any additional comment on that subject in that thread, but it's the same on Steam. Unlike Steam, though, we at least get Unreal 1 and 2 here.
It probably has to do with censorship, as the german retail version of UT 2004 was censored (it didn't include blood and gibbing), whereas there is likely only one international, uncensored digital version, which thus isn't allowed to be sold "in Germany".

Thank you for your answers, I've managed to buy it in the browser via VPN.

Then enjoy the game. :)