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;
     }
 }