The physenv library allows you to control the physics environment created by the engine, and lets you modify constants such as gravity and maximum velocity.
physenv.AddSurfaceData( string properties )
Adds surface properties to the game's physics environment. The game has a limit of 128 surface properties - this includes properties loaded automatically from surfaceproperties.txt. Due to this, there's only a small amount of open slots that can be registered with GMod's provided surfaceproperties.txt.Issue Tracker: 2604
Returns the air density used to calculate drag on physics objects. The unit is in kg/m^3.
Gets the gravitational acceleration used for physics objects in source_unit/s^2.
physenv.SetGravity( Vector gravAccel )
Sets the gravitational acceleration used for physics objects. Does not work on players.