Tried removing the changes but got an error …
Canceled: File is empty
When trying to save (or similar).
I saved it by replacing the changed stuff with a comment like this…
#Comment here
Which then got my DE back up after a reboot so I could look around, then found this…
systemctl revert gdm.service
which is supposed to reset the unit changes properly. After another reboot all is still working and loading up OK (although I had to reset my background wallpaper - odd).
I then tried the concept again with some minor changes following some Google trawling…
[Service]
ExecStartPre=-/bin/sleep 1
The - is supposed to make it so it doesn’t matter if it fails (so this may just be doing nothing) and I wasn’t sure if the duplication of ExecStart=/usr/bin/gdm was problematic at all.
It boots, whether I actually effectively achieved anything or not is a completely different question…