Spherical Gravity

Well, you need to ask one question at a time here. You also need to be a bit more specific on your needs. I'm not sure what you're asking as far as adding gravity to a sphere. If you're talking about adding a gravitational "pull" to a sphere, like a "planet", you could try something as simple as placing all the rigidbodies out in "space" into an array or list. Then in a for statement, get the distance between your planet and each body out in space. Make a range, in this example is 100, and have the planet pull those physical bodies toward it if they are close enough. I won't get into anything to detailed, or smooth functioning, because you weren't in any way specific. Try this out, break down the code, and look up each piece of it in the Unity Script Reference if you are not sure what it means. As for your terrain question, ask it as a separate question after you work this one out.

 //Example.cs
 
 using UnityEngine;
 using System.Collections;
 
 public class Example : MonoBehaviour
 {
     Object[] RBs;
     float range = 100.0f;
     float stopDistance = 10.0f;
     
     void Awake()
     {
         RBs = GameObject.FindObjectsOfType (typeof(Rigidbody));    
     }
     
     void Update()
     {
         foreach(Rigidbody RB in RBs)
         {
             float dist = Vector3.Distance(RB.transform.position, transform.position);
             if(dist < range && dist > stopDistance)
             {
                 RB.position = Vector3.MoveTowards(RB.position, transform.position, Vector3.Distance(RB.transform.position, transform.position) * Time.deltaTime);
             }
         }
     }
 }