How to add different gravity on spawning objects?

Im working on a project that I want to create a power up effect whenever the button "Q" is pressed, I have the animation working and the character, I also have the spawning objects around my player that I want to spawn (See Figure below) My question is how to add different gravity on each rock (spawning object).

Here is the script that I'm currently using.

/* Public Variables Declaration */
public Transform spawn_LocationForSmall;
public Transform spawn_LocationForMedium;
public Transform spawn_LocationForLarge;
public GameObject smallRock_Prefab;
public GameObject mediumRock_Prefab;
public GameObject largeRock_Prefab;

/* Private Variables Declaration */
private GameObject[] smallRocks_List;
private float posX, posY, posZ;

private bool smallCount = false;
private bool mediumCount = false;
private bool largeCount = false;

private bool small_CheckPos = false;
private bool medium_CheckPos = false;
private bool large_CheckPos = false;




void Start() {

    //smallRocks_List = GameObject.FindGameObjectsWithTag("smallRock");

    Create_Small_Rocks();

    Create_Medium_Rocks();

    Create_Large_Rocks();


}

 private void Create_Small_Rocks(){

    for(int i=0; i<=20; i++){

        small_CheckPos = false;
        posX = this.transform.position.x + Random.Range(-3.0f, 3.0f);
        posY = this.transform.position.y + Random.Range(-3.0f, 3.0f);
        posZ = this.transform.position.z + Random.Range(-3.0f, 3.0f);

        if(posX > 3f && posY > 3f){

            small_CheckPos = true;
        }

        if (small_CheckPos == true) {

            Vector3 newPos = new Vector3(posX, posY, posZ);

            GameObject createdObject = GameObject.Instantiate(smallRock_Prefab, 
                newPos, spawn_LocationForSmall.rotation) as GameObject;

            createdObject.transform.parent = spawn_LocationForSmall.transform;
        }

    }
    smallCount = true;
}
 /* the other two functions are similar to this */