gnome-mplayer-1.0.4 unconditionally depends on x11-libs/gtk+:3 and unconditionally calls configure with --enable-gtk3. Both should depend on the gtk3 USE flag (gnome-mplayer would be the only program pulling in gtk3 on my system, and I have no intention to have both or switch any time soon from gtk2).
*** This bug has been marked as a duplicate of bug 374057 ***