That implies to me you’ve converted your Ubuntu Server system into a Desktop system, thus the system will suspend on inactivity (eg. no mouse & keyboard movement etc), as Desktop defaults differ to Server defaults.
GDM is the GNOME Display Manager; it’s not used by Server installs, being part of the GNOME Desktop. If you do have desktop packages installed (you mention only Server), your system also now has desktop settings that use different configurations.
(If it’s not suspending; the GNOME suspend call on inactivity was possibly blocked by your server config)