On RHEL 7.6, gdm-3.28.2-2.el7 and systemd-219-57.el7 trigger this error:
systemd-udevd: invalid key/value pair in file /usr/lib/udev/rules.d/61-gdm.rules on line 2, starting at character 175 ('[')
It works fine with a newer systemd-udevd (eg., Fedora 28 with systemd-238).
probably fixed upstream, devack+
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.