Comment # 14 on bug 1089287 from Stefan Dirsch

Not sure, but gdm is running the Xserver on which the greeter runs as gdm user
and the Xserver on which runs the Xsession as regular user.

gdm       1427  0.3  2.2 463764 41524 tty7     Sl+  15:01   0:00 /usr/bin/X vt7
-displayfd 3 -auth /run/user/464/gdm/Xauthority -background none -noreset
-keeptty -verbose 3
tux       1922  0.2  2.2 464060 41660 tty2     Sl+  15:02   0:00 /usr/bin/X vt2
-displayfd 3 -auth /run/user/1000/gdm/Xauthority -background none -noreset
-keeptty -verbose 3

I don't believe NVIDIA has special requirements here.