Installed the latest gdm-40.0. Failed to start on missing plymouth service files and "failed to locate executable /usr/libexec/gdm-wait-for-drm: No such file or directory". Added plymouth use flag to eliminate the missing plymouth service file, but gdm still fails on second error.
Strange. gdm-wait-for-drm was added by gnome-base/gdm/files/gdm-CanGraphical-wait.patch which also adds it to the .service file. That patch isn't applied in v40.0. Not sure how this could happen... Oh, I wonder if you need to run systemd daemon-reload because it's trying to run the old service file?
Although rebooting my machine would reload the systemd daemon, your comment got me looking for a service file in my /etc/systemd/system directory. I found a gdm.service file there (don't recall how it got there) and deleted it. Problem solved. Thanks for the insight.