I don't fully understand what you are saying.
You set GDM to be your Display Manager?
How did you install Gnome?
You only have Gnome installed as a Desktop Environment?
I can't remember if it would be normal in that case to have just "default" as a choice - which would/should be gnome...
Did you switch to openrc?
the setup is a litte different there:
What is it you expect to see vs. what you do see?