Toggle Gravity on button press

I'm creating a side scroller where the Gravity of the player is a gameplay mechanic.

The initial gravity value is negative (falling downwards) and when the player presses the G key, the gravity changes to positive (floating upwards).

The script toggles a boolean value on/off when the G key is pressed. This part of the code works.

If the boolean is false(off) then the gravity should be -

if the boolean is true (on) then the gravity is equal to a antigrav variable (+gravity).

I can get the gravity to toggle so that the players gravity changes to positive, but I cannot change it back. Even though the boolean variable continues to toggle on/off.

 [RequireComponent (typeof (Controller2D))]
 public class Player : MonoBehaviour {
 
     public float jumpHeight = 4;
     public float timeToJumpApex = .4f;
     float accelerationTimeAirborne = .2f;
     float accelerationTimeGrounded = .1f;
     float moveSpeed = 6;
 
     public bool antigravTrue = false;
     float antigrav;
     float gravity;
     float jumpVelocity;
     Vector3 velocity;
     float velocityXSmoothing;
 
     Controller2D controller;
 
     void Start () {
         controller = GetComponent<Controller2D> ();
         gravity = -(2 * jumpHeight)/Mathf.Pow (timeToJumpApex, 2);
         antigrav = (2 * jumpHeight)/Mathf.Pow (timeToJumpApex, 2);
         jumpVelocity = Mathf.Abs(gravity) * timeToJumpApex;
         print ("Gravity: " + gravity + " Jump Velocity: " + jumpVelocity);
 
     }
 
     void Update(){
 
         if (controller.collisions.above || controller.collisions.below){
             velocity.y = 0;
         }
 
         Vector2 input = new Vector2(Input.GetAxisRaw ("Horizontal"), Input.GetAxisRaw ("Vertical"));
 
         if (Input.GetKeyDown(KeyCode.Space) && controller.collisions.below){
             velocity.y = jumpVelocity;
         }
 
         if (Input.GetKeyDown(KeyCode.Space) && controller.collisions.above){
             velocity.y = -jumpVelocity;
         }
 
         if(Input.GetKeyDown(KeyCode.G)){
             antigravTrue = !antigravTrue;
 
         }
 
         if(antigravTrue){
             gravity = antigrav;
         } 
 
         if(!antigravTrue){
             gravity = gravity;
             print ("its false");
         }
 
     
 
         float targetVelocityX= input.x * moveSpeed;
         velocity.x = Mathf.SmoothDamp(velocity.x, targetVelocityX, ref velocityXSmoothing,(controller.collisions.below)?accelerationTimeGrounded:accelerationTimeAirborne);
         velocity.y += gravity *Time.deltaTime;
         controller.Move(velocity * Time.deltaTime);
     }
 }