So this is what needs to be in a .desktop file, I've used my icewm.desktop file as an example. (these go in /usr/share/xsessions btw)
[Desktop Entry]
Encoding=UTF-8
Name=IceWM
Comment=This is the window manager IceWM
Exec=/usr/bin/icewm-session
Icon=icewm.xpm
Type=Application
That's it, this should make it pop up in gdm/kdm/whatever. It doesnt need to be executable or anything so just make the file /usr/share/xsessions/icewm.desktop and copy and paste the stuff above into it. The file gets a bit more complicated for stuff like Xfce, KDE, etc, but this is the gist of it.
Good luck, ttyl!