ckodadek: In GOG Galaxy 2.0, on my system only UIltima VII Part 2 Serpent Isle shows up as owned. If I install and hit play Ultima VII The Black Gate runs. It seems there is some issue with how Galaxy is seeing these 2 titles.
I am having the exact same problem just as you described.
I logged into the GOG.com website and found Ultima VII Black Gate + Forge of Virtue as one game AND Ultima VII Serpent Isle + The Silver Seed as a second game. Each game having its own offline downloads and the Download and Install Now option.
I used the offline installer file for each game and each install created 2 separate folders for each game. As well as 2 separate Launch executables within those folders. Launching the game(s) from it's corresponding folder, rather than trying to use Galaxy, launched each game as a stand alone game. Each game having different opening "movies".
At least I do have each game now and I hope you can do the same as I did but I agree with you, Galaxy isn't picking up Ultima VII Complete Edition. Unless I'm missing something this really needs to be fixed.