I have a two seat multi-seat setup.  When one seat is active and the other
is not (under GDM control), then after about twenty minutes the computer
goes into suspend mode, even in the middle of typing a sentence on the
active seat.

The system was not doing this till a software update came through about a
week ago.  Hence why I think this is a gnome issue, and probably a GDM
issue.

Note this happens even though on the active seat I set the under
settings->Power->Automatic
Suspend to OFF.