I’ve been struggling with getting gdm on xserver to work with a simple HyperV install and it would never come up after boot (eternal blinking cursor on tty7). Upon checking logs with journalctl, I saw a clear permission issue with getting access to /dev/fb0. Adding the gdm user to the video group solved the problem, so hopefully this will help other folks too:
users.users.gdm { extraGroups = [ "video" ] }
What I don’t quite understand is why the combination of xserver + gdm + gnome requires both the display manager user and the logged user to be on the video group and the combination of, say, xserver + lightdm + gnome doesn’t for either user. Must be the way lightdm initially grabs and hands off access to the display device from its user to the logged user, which is different from gdm. But I’m only speculating here.