I bought Ultima VII The Complete Edition and there appears to be a problem with accessing Serpent Isle content. In my GoG library I have two Ultima VII related items. One is "Ultima VII Serpent Isle + Silver Seed" however this takes me directly to The Black Gate. There is also an item called "Ultima VII The Complete Edition" but I can't play it and the install button is grayed out. I have a desktop icon for The Black Gate but not one for Serpent Isle. What happened and how can I launch Serpent Isle?

I just finished The Black Gate and would like to continue with the story, but until this is resolved I will be unable to do so.

This doesn't answer your question, but technically, to continue the story, you should play Ultima Underworld II next as it takes place between Black Gate and Serpent Isle.

If you're using Galaxy, then it seems that there's a UI-control that determines whether The Black Gate or Serpent Isle is launched. The following thread describes how to toggle between the two:

https://www.gog.com/forum/ultima_series/ultima_7_complete_collection/post7