Like Taser said, gravity is already build in to the Platform behavior. You can change its value in the properties panel, or at run time (if you want stronger or lighter gravity, you can also change the Angle of Gravity). The default value is 1500. As long as there is a value for gravity, everything will happen automatically. There is no need to do anything in the OnJump() event (other than change animations, or things along those lines).