Hey, when I wanted to change my default session for gdm I simply did as you did and went into
/etc/X11/gdm/Sessions then I opened up Default and default in emacs, then simply changed the line to the one found in the Gnome file (also in /etc/X11/gdm/Sessions). I backed up all the files for the original directory, then saved everything (as root of course) and rebooted. I don't know if you already did this, or if I'm telling you things that are redundant, but that's what I did and it seemed to fix the problem, as my default is now Gnome and I don't have to manually select it from sessions any longer.
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