Ok, I got that working, but I don't know the projectile name of the gravity rocket apprently.
This is what I put in the server.cs:
ShootOnClick_Projectile[0]=RocketGravity;
This is the error I get:
Object 'RocketGravity' is not a member of the 'GameBaseData' data block class
Add-Ons/Vehicle_Go_Kart/Support_shootOnClick.cs (99): Register object failed for object (null) of class Projectile.
Set::add: Object "0" doesn't exist