I just upgraded to 15.10, and everything went smoothly. One of the first things I did was install gnome-shell, and switch from lightdm to gdm. However, when I restarted, after an initial purple Ubuntu load screen, I got a grey/black screen that never went away. I switched back to lightdm and everything worked fine again. Is this a problem with gdm in 15.10, or is there something I am not doing/doing wrong? Thank you!
Update:
I have been following this bug report: bugs.launchpad.net/ubuntu/+source/gdm/+bug/1500673 originally linked below by @ulkeshkosh, and there is some new activity there including a suggested fix that worked for one user. I tried this fix, and it didn't work for me, nor did it work for the other commenters, but it might work for someone else visiting this post.
However, a word of warning: one commenter mentions this line in /etc/gdm/custom.conf:
[daemon]
# Uncomment the line below to force the login screen to use Xorg
#WaylandEnable = false
the commenter does not suggest modifying this line in any way, but being an inexperienced and curious Linux user, I decided to see what happens when I uncommented it, and the results were quite a pain to recover from, so I would suggest that you not experiment in this way.
gnome-shell, like the lock screen and screen saver not working (that is, usinglightdmwithgnome-shelldoesn't always go smoothly) – elethan Oct 25 '15 at 13:40