Rotate Gravity 90 degrees for one object.

How do I rotate a player character’s gravity 90 degrees on button press? An example of what I want it to do is when you press page down, the gravity of ONE object / character only rotates 90 degrees clockwise. I have tried almost every possible way to get it to work. Please help ASAP.
Below is what I currently have as a script that is attached to the character:

var forceToAdd : Vector3 = Vector3(5,5,0);
var obj:GameObject;
private var other:RotateObject;
var rotating : boolean = false;
var changegravityminus :boolean = false;
var changegravityplus :boolean = false;

function Start() {
	other = obj.GetComponent(RotateObject);
}

function Update () {
	if (other.gravity == 1){
		transform.Rotate (0,0,0);
		rigidbody.AddForce (0, 0, 0);
		Physics.gravity = new
		Vector3(0f,-9.81f,0f);
	}
	
	if (other.gravity == 2){
		transform.Rotate (0,0,90);
		rigidbody.AddForce (0, 0, 0);
		Physics.gravity = new
		Vector3(-9.81f,0f,0f);
	}
	
	if (other.gravity == 3){
		transform.Rotate (0,0,90);
		rigidbody.AddForce (0, 0, 0);
		Physics.gravity = new
		Vector3(0f,9.81f,0f);
	}
	
	if (other.gravity == 4){
		transform.Rotate (0,0,90);
		rigidbody.AddForce (0, 0, 0);
		Physics.gravity = new
		Vector3(9.81f,0f,0f);
	}
	
	
	if(Input.GetKeyDown (KeyCode.PageUp)&& rotating==false){
		changegravityminus=false;
		rotating=true;
		transform.Rotate (0,0,-90);
		rigidbody.velocity = forceToAdd;
		Physics.gravity =Quaternion.Euler(0, 0, -90) * Physics.gravity;
		rotating=false;
		changegravityminus=true;
	}
	
	if(Input.GetKeyDown (KeyCode.PageDown)&& rotating==false) {
	rotating=true;
	transform.Rotate (0,0,90);
	rigidbody.velocity = forceToAdd;
	Physics.gravity =Quaternion.Euler(0, 0, 90) * Physics.gravity;
	rotating=false;
	}
}

if(Input.GetKeyDown(“Fire1”)){
yourDifferentGravityGO.rigidbody.useGravity = false;

    //a flag to refer to in fixed update so you can change it's behaviour
    yourDifferentGravityGO.isGravityDifferent = true;

    //then apply a constant force in Fixed Update in the direction you want
}

Turn the default gravity off, apply a constant force.