I made a gravity type of thing for one of my games.
You used a "gravlifter" to move crates and slam them into the ground, smashing rats.

Anyway heres the relevent function
Code:

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function gravGunMain(){
	var myTemp;
	sfxBuzz = ent_playloop(gravEnt,gravBuzz,600); //start buzzing noise at gravEnt
	sfxElectroHumm = snd_loop(electroHumm,25,0); //start underlying electric humm
	while(effectCount < 6){
		ent_create("magma_tirc.bmp",gravEnt.x,gravFx); //create effects
		effectCount += 1;
	}
	while(gravSwitch == 1){
		phent_setdamping(gravEnt,90,30); //set linear damping high to help control
		//- slam gravEnt to ground
			if(mouse_right){
				if(myTemp == 0){
					myTemp = 1;
					phent_addvelcentral(gravEnt,vector(0,0,-1500));
				//	phent_addvellocal(gravEnt,vector(1000,0,0),vector(0,0,0));
	
				}
			} 
			else{ //return to hover position
				myTemp = 0;				
			}
			gravPointEnt.z = max(-350,min(gravPointEnt.z,-200)); //lock z range of gravPointEnt
				
		//- set and adjust final movement vector
			vec_lerp(gravForce,gravPointEnt.x,gravEnt.x,0.5);
			vec_sub(gravForce,gravEnt.x);
			
		//- add physics force to gravEnt
			phent_addvelcentral(gravEnt,gravForce);
		wait(1);	
	}
	snd_stop(sfxBuzz); //stop buzzing
	snd_stop(sfxElectroHumm); //stop humming
	phent_setdamping(gravEnt,10,30); //return to low damping for better free flight	
}

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gravPointEnt is an invisible entity that is controled with the mouse, this entity marks where the gravEnt should be.
gravEnt is the entity affected by gravity.
gravSwitch is the toggle, == 1 gravity gun is on, == 0 freeflight