Did install Funtoo on 2 pc's with gnome-3.14 DE, different hardware but both of them gdm started with a black screen and a cursor in the to left corner, nothing wrong in the logs even with debug logging enabled.

Both systems are stable build profile, desktop flavor with gnome and hardened mix-ins extra. Kernel, sys-kernel/debian-sources-3.16.2:3.16.2 USE="binary -rt".

After hint from grosjean on irc I started to keyword and update some libraries of gnome/xorg. On the first machine (laptop) I accepted keywords for (and the required dependency's):
=x11-libs/libXi-1.7.4 ~amd64
=x11-libs/libxcb-1.11 ~amd64
=x11-proto/xproto-7.0.26 ~amd64
=x11-libs/libICE-1.0.9 ~amd64
=x11-libs/libXext-1.3.3 ~amd64
=x11-proto/inputproto-2.3.1 ~amd64
=x11-proto/xproto-7.0.26 ~amd64
=x11-base/xorg-server-1.16.1-r1 ~amd64
=x11-apps/mesa-progs-8.2.1 ~amd64
=x11-libs/libXfont-1.5.0 ~amd64

After that is started to work, now on the second machine I tried 1, 1 to find the actual problem. After the upgrade of x11-libs/libXi-1.7.4 it also started to work, but not 100% sure if it was that because I forgot to reboot the pc and confirm it's still not working before upgrading libXi. Will try later to replicate and confirm.