So I made a GOGmix and I am wondering if I am allowed to advertise it in the forums. Because I like to know what people thinks about the effort I put and if its a good GOGmix.

The mix in question is that I talk about games "that are heavily inspired by Ultima Underworld"

http://www.gog.com/mix/games_inspired_by_ultima_underworld?ccr=y

Sure you can ;-)
There is even a "promote your gogmix" thread:
http://www.gog.com/forum/general/the_gogmix_central_thread_sharepromote_your_gogmixes_here/