For instance 10VX would make a particle to right of the screen or 99999999999XV would blast everything out of its way by creating pressure and veolocity
!set vx grvt [speed] AND !set vy grvt 0
You can use the property tool and set its VX [Veolocity X] or its VY [Veolocity Y]
how did you make grvt move in one direction?