A lot of use flags should probably be used in the gdm ebuild, now that X is modular. For example, dmxproto and libdmx should only be pulled in if the dmx flag is set. libXinerama should only be a dependency if xinerama is set. Maybe even make libXdmcp optional by a xdmcp flag. Since the configure for gdm decides on a lot of things automatically, the only other changes that have to be made is to change --with-xdmcp to $(use_with xdmcp).
we will not add use flags unless there are *real* configure switches. re-open if you have patches, thanks.