Instructions
Decide to what degree you want to reduce gravity. Pick a number from 300 to 1,000, where 300 is normal gravity and 1,000 is free-floating.
Set the game to console mode by pressing the tilde key, or ~.
Enter the following command in the command prompt where "xxxx" is the number you chose in Step 1: g_gravity=xxxx
Press Enter. The next time you jump, your character will experience the new level of gravity.