Gdm-2.26.1 doesn't start when using xorg-x11-7.4-r1 (works fine with xorg-x11-7.4). X itself works, I can start it (and desktop) with startx, but gdm refuses to co-operate.
emerge --info, emerge -pv gdm xorg-server and logs of X (/var/log/Xorg.0.log) and gdm (/var/log/gdm/:0.log) please
Sorry, I almost forgot to answer to this... and I'm a bit embarrassed... I seem to be too used to revdep-rebuild; when it doesn't detect anything, I assume nothing is broken. To cut the long story short: recompiling gdm fixed the issue :)