NOTE: This is unsafe

Temporarily disable X server’s access control.

Disable for only local:

$ xhost +local:
non-network local connections being added to access control list

Disable for any host:

$ xhost +
access control disabled, clients can connect from any host

Run an X program as another user.

$ sudo -u gdm dbus-launch gnome-control-center

If you also need to change the user’s shell.

$ sudo -u gdm -s /bin/bash -c "dbus-launch gnome-control-center"

Renable X server’s access control.

Only local

$ xhost -local:
non-network local connections being removed from access control list

Any host

$ xhost -
access control enabled, only authorized clients can connect