I screwed up gdm3, can't recover

["Thomas H. George" <lists@tomgeorge.info>]

OK, my fault.

A new jessie installation. Display problem so I tried to switch to xdm.
When this didn't work I tried to switch back to gdm3. No Go:

gdm3.serviceJob for gdm.service failed.

Looked for solution in man systemctl, found reset-failed command. Tried

systemctl reset-failed gdm.service

exit code was 0 but problem was not fixed.

tried apt-get install --reinstall gdm3

problem not fixed.

What to do? Reinstall jessie? Continue experimenting with systemctl
commands? 

Any suggestions?