How to keep a gameobject on the ground ?

Just add a rigidbody to your character, should work... not sure if rigidbody2d supports gravity. If it doesn't, then make a function, where your y position gets decreased over time. For example:

 public class BallController : MonoBehaviour {
 
 public float gravity;
 
 void Update () {
 Gravity();
 }
 
 void Gravity () {
 
 transform.position.y -= gravity;
 
 }
 }

Hope it helps!