GDM changes the permissions of some directories at runtime[1] which causes warnings from ALPM. I adjusted the permissions in the PKGBUILD to match GDM's preferences since the default permissions in Makefile.am also don't match[2]. (I should report this upstream, too.) I also changed the owners of the directories directly in the PKGBUILD to get rid of that chown hack in the install script and to avoid warnings with pacman -Qkk.

The groupadd and useradd commands were missing the --system parameter which causes the user to be created without aging information and which will not create a home directory regardless of the setting CREATE_HOME in /etc/login.defs which is controllable by Arch users. The passwd -l is unnecessary. New users are created with locked password by default.
In post_remove a true was added to always give a success return value, even if userdel removes the gdm group because of USERGROUPS_ENAB in /etc/login.defs set to yes and thus getent gives an error code.

At last I removed the leading slashes in the install script. There doesn't seem to be a rule for this but most scripts don't have it. If you want a patch without removing the leading slashes, please tell me.

[1] https://git.gnome.org/browse/gdm/tree/daemon/main.c#n180
https://git.gnome.org/browse/gdm/tree/daemon/gdm-launch-environment.c#n463
[2] https://git.gnome.org/browse/gdm/tree/data/Makefile.am#n279