Enlightenment seems to be the only Window Manager that is
fully configured to work out-of-the-box. I should think,
with many other WM's like AfterStep and WindowMaker now
nearly at 100% GNOME compliance, these could be added as
choices to the WM selector as well.
A Gnome-compliant window manager (like WindowMaker and
Enlightenment) inserts itself into the Window Manager selection in
Gnome control-center when installed.
I.E. when the aforementioned window managers _are_ Gnome compliant,
they will be there.