how to listing all game objects in GUI.Box?

First issue is that `objects` should be a string array; the declaration looks like this:

 public string[] objects;

Next, if you're looking for GameObjects in scene, cast them as such. This line:

 object[] obj = GameObject.FindSceneObjectsOfType(typeof (GameObject));

should look like this:

 GameObject[] obj = GameObject.FindSceneObjectsOfType(typeof (GameObject)) as GameObject[];

Then, in your `Start` method, the foreach loop should assign the name to the proper index within that string. I think it's easier to just use a for loop. Something like this:

 // Set length of string array to match the length of obj array.
 objects = new string[obj.Length];
 for(int i = 0; i < obj.Length; i++)
 {
     objects[i] = obj[i].name;
 }

Now that you have an array containing the name of all your gameObject's names, iterate through that list in your OnGUI function. You'll need to repaint each label every frame, and it looks like you had a decent start there. Just add a for loop instead of trying to initialize `i` every time OnGUI is called (it will always be 1 in your example code).

 void OnGUI()
 {
     Rect sizeBox = new Rect(Screen.width/2-1, Screen.height/2-150, 300, 100);
 
     // Calling ToString() on an array object won't format all members, so build 
     // our own string and format it ourselves.  We use StringBuilder instead of
     // string += "" because StringBuilder is much faster when concatenating lots
     // of strings.  That, or move this to the Start() method and only call it once.
     // In fact, you should probably do that anyways.
     StringBuilder allGameObjects = new StringBuilder();
     for(int i = 0; i < objects.Length; i++)
         allGameObjects.Append(objects[i] + "\n");

     GUI.Box(sizeBox, "Available Objects : \n" + allGameObjects.ToString());
 }

And here is everything put together.

 using UnityEngine;
 using System.Collections;
 using System.Text;
 
 public class SceneObjects : MonoBehaviour {
 
     string gameObjectList;
 
     public void Start()
     {
         GameObject[] obj = GameObject.FindSceneObjectsOfType(typeof (GameObject)) as GameObject[];
         string[] objects;
 
 
         objects = new string[obj.Length];
         for(int i = 0; i < obj.Length; i++)
             objects[i] = obj[i].name;
 
         StringBuilder allGameObjects = new StringBuilder();
         for(int i = 0; i < objects.Length; i++)
             allGameObjects.Append(objects[i] + "\n");
 
         gameObjectList = allGameObjects.ToString();
     }
 
     void OnGUI()
     {
         Rect sizeBox = new Rect(Screen.width/2-1, Screen.height/2-150, 300, 100);
 
         // Calling ToString() on an array object won't format all members, so build 
         // our own string and format it ourselves.  We use StringBuilder instead of
         // string += "" because StringBuilder is much faster when concatenating lots
         // of strings.  That, or move this to the Start() method and only call it once.
         // In fact, you should probably do that anyways.
 
         GUI.Box(sizeBox, "Available Objects : \n" + gameObjectList);
     }
 }