I just posted a bug report on https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=896083
The solution to the problem is further down in the bug report.
gdm3’s default dconf energy settings suspend the machine after 20 minutes.
This is independent of the power settings made by an unprivileged user within a Gnome login session.
While this could be forgiven on a locally accessible desktop machine, it also suspends remote/headless machines (e.g. in a data center). Activity on a SSH terminal or VNC connection does not prevent this issue. Having no easy way to re-wake remote machines, this may create highly inconvenient situations for administrators. In addition, unexpected suspension may also have disastrous consequences, depending on the use of the machine.
To reproduce, install task-gnome-desktop and wait for 20 minutes on a machine which supports power management.
The offending settings can be printed to the console. As superuser:su -s /bin/bash Debian-gdm unset XDG_RUNTIME_DIR dbus-launch gsettings get org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type dbus-launch gsettings get org.gnome.settings-daemon.plugins.power sleep-inactive-ac-timeout
This prints ‘suspend’ and ‘1200’, respectively.
For quicker reproduction of the problem, reduce the timeout to 2 minutes:dbus-launch gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-timeout 120
Then reboot and wait 2 minutes.
To turn off suspension, set:dbus-launch gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type nothing