According to Plex; What is GDM?
The wiki is not clear, and I dug through at least 5 pages each of a couple forum searches.
Google, of course, brings up millions of hits for Gnome Display Manager, and it's not helped by narrowing to Plex or XBMC.
Ubuntu 14.04.1 LTS x86_64 | sabnzb+, sickbeard
i3-4330 OSX86 (edits only through clover) 10.9.4 Plex / Backup server / File Server
ASUS RT-AC66U w/Stock firmware (physical connection to the server)
2x iPhone 4 w/Plex app, iPad Air 2 w/Plex app, iPad 1st Gen w/Plex app
I've successfully run Two (then three) Plex Instances on the Same Server. Click here to learn more: https://raspberrypihtpc.wordpress.com/2015/10/01/two-or-more-plex-server-instances-on-the-same-server/
*Constantly happy with how well Plex works, and where it's headed