From the hipnotic entities, this trigger will change the gravity experienced by the player.
Unlike for the worldspawn key or the console command, the key “gravity” here is in the form of a percentage. A value of 100 is normal gravity, as is the default of 0. A value of 50, therefore, would produce half normal gravity.
Note that this can be used to create specific ‘lo-g’ areas of a map, like in the SoA start map, but only affects the player. Monsters and coop team-mates will not be affected.