I recently installed debian with xfce. Once installed, I upgraded to Debian testing and installed awesome, then rebooted. When I click the list of session options at GDM's login screen, the only options are xfce and "default" (xfce). Awesome has a desktop file in /usr/share/xsessions, but GDM can't find it for whatever reason.
Have you tried following the official guide? You have to create/modify multiple files for this to work.