So, I've been having trouble finding a definitive answer on how to do this. I've compiled and installed a window manager that is not available in the ubuntu package repositories, and now I'm attempting to add it to the gdm options menu. What's the "Ubuntu" way to do this? I'm sure there must be a 'most correct' approach. Anyone have any clues, or links to websites? I've seen a few answers in the ubuntu forums, but no-one really had a clear, "This is the right way to do it" that I found.
By gdm options menu, do you mean the list at the bottom of the login screen ?
If yes, then, as far as I know, the "correct approach" for adding a window manager to the list of session options presented to user at login time is to add a .desktop file to /usr/share/xsessions. Take an existing .desktop file there as starting point (say, gnome.desktop) and modify fields like Exec, TryExec, Icon to suit your new WM.
Since this is so obvious (by that I mean covered in the tutorials of many WM's I have tried to add), I think I haven't understood your question well. Can you confirm if you have tried this ?