Xmonad is a dynamically tiling X11 window manager that is written and configured in Haskell.
Emerge gnome-base/gnome, x11-wm/xmonad and x11-wm/xmonad-contrib.
emerge gnome xmonad xmonad-contrib
A nice xmobar surrogate is the xmonad-log-applet for gnome. A ebuild currently resided within the haskell overlay.
layman -a haskell
emerge xmonad-log-applet --autounmask-write
Modify your xmonad.hs with the code provided by the sample xmonad.hs. You can finde sample configurations in the Xmonad config archive, also the configuring xmonad manual is a good starting point.
Note: A non-functional xmonad.hs may leave you with a desktop without any gnome-panels. You have to restart xdm manually in that case and check ~/.xsession-errors for errors
Add the XSession
Comment=Gnome and Xmonad
#xcompmgr -c -f -F &
chmod 755 /usr/sbin/xmonad-gnome-session
Warning: Enabling composite effects can result in unexpected behavior on some software/hardware combinations. See Bug 339716 for an example.
You can enable composite effects by uncommenting the xcompmgr line above.
Check support with
xdpyinfo | grep Composite
and emerge x11-misc/xcompmgr