Instructions
Begin the game. Select "Options" in the game's main menu.
Select the "Keyboard/Mouse" option in the options menu, then select the "Allow Developers Console" option. Begin playing the game.
Press the "~" option during play, then type the following text into the prompt:
Sv_Cheats 1
This will enable cheat mode.
Type the following into the prompt:
ent_fire !self addoutput "gravity 0.10
This will lower the game's gravity.