Disable GDM suspend on lock screen

That's because of the idle-delay setting. To change it you'll have to alter the corresponding dconf key (and do that as the gdm user):

switch to a VT (e.g. Ctrl+Alt+F3), login as root and run:
```
su - gdm -s /bin/sh
```
to switch user to gdm.

then run:

export $(dbus-launch)

and set idle delay to 0 (which translates to never):

GSETTINGS_BACKEND=dconf gsettings set org.gnome.desktop.session idle-delay 0

run exit or hit Ctrl+D to return to root account.

reboot your machine or restart the display manager:
```
systemctl restart gdm
```

Just as an expansion to don_crissti's answer:

Step 2 didn't work for me. I am on Gnome/gdm 3.28 and I don't have org.gnome.desktop.session, but this one worked for me:

GSETTINGS_BACKEND=dconf gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type 'nothing'

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