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.
Last I looked, Debian's awesome package is kind of broken. Go look at
/usr/share/xsessions/awesome.desktop and see if there's a line in there reading
NoDisplay=true. If there is, change it to
false, or delete the line entirely.
Have you tried following the official guide? You have to create/modify multiple files for this to work.