I just upgraded to 15.10, and everything went smoothly. One of the first things I did was install
gnome-shell, and switch from
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!
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
# 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.