From Ubuntu (Unity) 16.04, 16.10 and 17.04, I enabled -proposed updated and installed gnome-shell and was not asked whether to pick gdm3 or lightdm. I logged out of Unity and logged into GNOME successfully.
gnome-shell 3.18.5-0ubuntu0.3 (16.04) gnome-shell 3.20.4-0ubuntu3 (16.10) gnome-shell 3.24.2-0ubuntu0.1 (17.04)
From Ubuntu (Unity) 16.04, 16.10 and 17.04, I enabled -proposed updated and installed gnome-shell and was not asked whether to pick gdm3 or lightdm. I logged out of Unity and logged into GNOME successfully.
gnome-shell 3.18.5-0ubuntu0.3 (16.04)
gnome-shell 3.20.4-0ubuntu3 (16.10)
gnome-shell 3.24.2-0ubuntu0.1 (17.04)