sudo systemctl status gdm3
● gdm.service - GNOME Display Manager
   Loaded: loaded (/lib/systemd/system/gdm.service; static; vendor preset: enabled)
   Active: active (running) since Mon 2018-06-18 01:15:04 EDT; 3min 30s ago
  Process: 1337 ExecStartPre=/usr/share/gdm/generate-config (code=exited, status=0/SUCCESS)
 Main PID: 1345 (gdm3)
    Tasks: 3 (limit: 4915)
   CGroup: /system.slice/gdm.service
           └─1345 /usr/sbin/gdm3

Jun 18 01:15:04 little-black-box systemd[1]: Starting GNOME Display Manager...
Jun 18 01:15:04 little-black-box systemd[1]: Started GNOME Display Manager.
Jun 18 01:15:04 little-black-box gdm-launch-environment][1354]: pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
Jun 18 01:15:04 little-black-box gdm-launch-environment][1354]: pam_unix(gdm-launch-environment:session): session closed for user gdm
Jun 18 01:15:04 little-black-box gdm3[1345]: GdmDisplay: display lasted 0.382228 seconds
Jun 18 01:15:04 little-black-box gdm3[1345]: Child process -1382 was already dead.
Jun 18 01:15:04 little-black-box gdm3[1345]: Child process 1354 was already dead.
Jun 18 01:15:04 little-black-box gdm3[1345]: Unable to kill session worker process
Jun 18 01:15:04 little-black-box gdm-launch-environment][1444]: pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
william@little-black-box:~$ sudo systemctl stop gdm3
william@little-black-box:~$ sudo systemctl status gdm3
● gdm.service - GNOME Display Manager
   Loaded: loaded (/lib/systemd/system/gdm.service; static; vendor preset: enabled)
   Active: inactive (dead) since Mon 2018-06-18 01:18:52 EDT; 2s ago
  Process: 1345 ExecStart=/usr/sbin/gdm3 (code=exited, status=0/SUCCESS)
  Process: 1337 ExecStartPre=/usr/share/gdm/generate-config (code=exited, status=0/SUCCESS)
 Main PID: 1345 (code=exited, status=0/SUCCESS)

Jun 18 01:15:04 little-black-box gdm-launch-environment][1354]: pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
Jun 18 01:15:04 little-black-box gdm-launch-environment][1354]: pam_unix(gdm-launch-environment:session): session closed for user gdm
Jun 18 01:15:04 little-black-box gdm3[1345]: GdmDisplay: display lasted 0.382228 seconds
Jun 18 01:15:04 little-black-box gdm3[1345]: Child process -1382 was already dead.
Jun 18 01:15:04 little-black-box gdm3[1345]: Child process 1354 was already dead.
Jun 18 01:15:04 little-black-box gdm3[1345]: Unable to kill session worker process
Jun 18 01:15:04 little-black-box gdm-launch-environment][1444]: pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
Jun 18 01:18:52 little-black-box systemd[1]: Stopping GNOME Display Manager...
Jun 18 01:18:52 little-black-box gdm3[1345]: GLib: g_hash_table_find: assertion 'version == hash_table->version' failed
Jun 18 01:18:52 little-black-box systemd[1]: Stopped GNOME Display Manager.
william@little-black-box:~$ sudo systemctl start gdm3
william@little-black-box:~$ sudo systemctl status gdm3
● gdm.service - GNOME Display Manager
   Loaded: loaded (/lib/systemd/system/gdm.service; static; vendor preset: enabled)
   Active: active (running) since Mon 2018-06-18 01:19:10 EDT; 1s ago
  Process: 3910 ExecStartPre=/usr/share/gdm/generate-config (code=exited, status=0/SUCCESS)
 Main PID: 3912 (gdm3)
    Tasks: 4 (limit: 4915)
   CGroup: /system.slice/gdm.service
           └─3912 /usr/sbin/gdm3

Jun 18 01:19:10 little-black-box systemd[1]: Starting GNOME Display Manager...
Jun 18 01:19:10 little-black-box systemd[1]: Started GNOME Display Manager.
Jun 18 01:19:10 little-black-box gdm-launch-environment][3916]: pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
Jun 18 01:19:10 little-black-box gdm-launch-environment][3916]: pam_unix(gdm-launch-environment:session): session closed for user gdm
Jun 18 01:19:11 little-black-box gdm3[3912]: GdmDisplay: display lasted 0.205813 seconds
Jun 18 01:19:11 little-black-box gdm3[3912]: Child process -3932 was already dead.
Jun 18 01:19:11 little-black-box gdm3[3912]: Child process 3916 was already dead.
Jun 18 01:19:11 little-black-box gdm3[3912]: Unable to kill session worker process
Jun 18 01:19:11 little-black-box gdm-launch-environment][3958]: pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
william@little-black-box:~$