/etc/X11/gdm/Sessions/gdm calls /etc/X11/xdm/Xsession
both of these are bash login shells.
this can cause /etc/profile et al being run twice -> $PATH
becomes very long!
I use gdm & I've taken the login off /etc/X11/xdm/Xsession this
works but my not suit xdm users.
"/etc/X11/gdm/Sessions/gdm" (I think you mean /etc/X11/gdm/Sessions/Gnome)
Currently this calls gnome-session directly, so I don't see any problem here.