This is the wiki page for the cvar
The command allows modifying the gravity in offline games. Normal gravity has a value of -650. It is negative because it acts downwards. Therefore, regular strength inverse gravity would be a value of 650.
When setting the gravity to 0, the game will revert back to default (-650) gravity. To actually get a zero gravity environment set the value to 0.0000001. This makes sure it's unequal to 0, yet so weak that it will not affect your car at all.
To change the gravity while in freeplay or other offline modes, open the console and type
Replace -400 with the desired value.