control object rigidbody individualy ?

In order not to change the gravity for all objects at once, it is necessary to implement your gravity for each object separately. This script does this:

 using UnityEngine;
 
 [RequireComponent(typeof(Rigidbody2D))]
 public class YourGravity : MonoBehaviour
 {
     // gravity when the object is in the positive x-axis
     public Vector2 gravity = Vector2.left * 9.81f;
     Rigidbody2D selfRigidbody2D;
 
     void Start()
     {
         // get the physical component
         selfRigidbody2D = GetComponent<Rigidbody2D>();
 
         // disable built-in gravity
         selfRigidbody2D.gravityScale = 0;
 
         // to make our gravity look smooth, turn on interpolation.
         selfRigidbody2D.interpolation = RigidbodyInterpolation2D.Interpolate;
     }
 
     void FixedUpdate()
     {
         if (transform.position.x > 0)
         {
             selfRigidbody2D.velocity += gravity * Time.fixedDeltaTime;
 
         }
         else if (transform.position.x < 0)
         {
             selfRigidbody2D.velocity -= gravity * Time.fixedDeltaTime;
         }
     }
 }

EDITED:

If you need to change the gravity for one object when you release the left mouse button, then the script must be changed:

 using UnityEngine;
 
 [RequireComponent(typeof(Rigidbody2D))]
 public class YourGravity : MonoBehaviour
 {
     // gravity when the object is in the positive x-axis
     public Vector2 gravity = Vector2.left * 9.81f;
     Rigidbody2D selfRigidbody2D;
 
     bool isClickedOnThisObject;
 
     void Start()
     {
         // get the physical component
         selfRigidbody2D = GetComponent<Rigidbody2D>();
 
         // disable built-in gravity
         selfRigidbody2D.gravityScale = 0;
 
         // to make our gravity look smooth, turn on interpolation.
         selfRigidbody2D.interpolation = RigidbodyInterpolation2D.Interpolate;
     }
 
     private void Update()
     {
         // Processing of keystrokes and mouse clicks
         // should always be done in the Update () function.
 
         //In this condition, we check if the player pressed
         //the given object and released the left mouse button.
         //If yes, then we change the gravity of this object.
 
         if (isClickedOnThisObject && Input.GetMouseButtonUp(0))
         {
             isClickedOnThisObject = false;
             gravity *= -1;
         }
     }
 
     void FixedUpdate()
     {
         // Physical calculations should always be done
         // in the FixedUpdate() function.
 
         selfRigidbody2D.velocity += gravity * Time.fixedDeltaTime;
     }
 
     private void OnMouseDown()
     {
         // Here we use the OnMouseDown() function, because
         // the OnMouseUp() function is called on all objects,
         // regardless of the location of the cursor.
 
         isClickedOnThisObject = true;
     }
 }