Heres a commented example for you...

Code:

---

 

  vec_set(temp, my.x); // makes it so temp is always starts at my.x

  temp.z -= 4000; // set how far to trace

  trace_mode = ignore_me+ignore_sprites+ignore_models; // sets the trace

  air = trace(my.x, temp); // see's if player will hit ground sometime



if(air > 3) //if the player is still in the air

{

force = -12 * time; // sets how fast you fall

}



else

{

force = 0; // makes sure you dont fall anymore

} 

---

So just have your player_speed.z value to equal force. Hope that helps