From 2.18 release, gnome-games package includes glchess, so I think that is better do add a block in the ebuild.
Done for gnome-games. Games: A blocker for >=gnome-extra/gnome-games-2.18.0 should probably be added to glchess too, assuming it stays in the tree. If you'd rather wait for 2.18.0 to get in the tree, that's fine with me, too.
done