I am using lightdm. gnome-shell 3.6 will crash on startup, if gnome-screensaver is not installed. See this bug's meta for respective bgo bug URL.
This should be reflected somewhere in ebuild dependencies.
Bug 449452 may be related.
I can confirm it.
Using lightdm too, after upgrading to 3.6 I couldn't login (gnome-shell crashing immediately) until I installed gnome-screensaver.
Please try with 3.8 (in that cycle gnome-screensaver was dropped entirely)