Unity GUI multiple grids?

Hello, I have 24 buttons in a grid that the player can select and use how they want, you are able to bring this grid up by pressing a button called "Items1". However I would like to have more buttons but not add them to this specific grid. Instead have another button saying "Items2" that when clicked brings up another grid of different buttons. I have already tried to do this and failed miserably, can someone help me out?

This is my script so far,

 var selGridInt : int = 0;
 var selStrings : String[] = ["Grid 1", "Grid 2", "Grid 3", "Grid 4"];
 var showInventory = false;
 
 public var prefabObject : GameObject;
 private var offset : Vector3;
 private var fwd : Vector3;
 private var clone : GameObject;
 var unit : float = 1.0f;
 var Sound : AudioClip;
 
 
 function Update () {
 
 fwd = transform.TransformDirection(Vector3.forward) * unit;
 offset = transform.position + fwd;
 if(Input.GetKeyDown(KeyCode.E)){
     
 audio.PlayOneShot(Sound);
 clone = Instantiate(prefabObject, offset, transform.rotation);
 }
 
 if(Input.GetKeyDown(KeyCode.I)){
 showInventory = !showInventory;
 
 }
 }
 
 var showItems1 = false;
 
 function OnGUI () {
 
 if(showInventory == true){
 GUI.Box (Rect (180,40,340,380), "Menu");
 
 if (GUI.Button (Rect (220,60,80,20), "Items1")) {
 showItems1 = !showItems1;
 }
 
 if (GUI.Button (Rect (400,60,100,20), "Quit Game")) {
 Application.LoadLevel(0);
 }
 
 
 if(showItems1 == true){
 selGridInt = GUI.SelectionGrid (Rect (200, 100, 300, 300), selGridInt, selStrings, 2);
 
 if((selGridInt == 0) && showInventory == true){
 prefabObject = (Resources.Load("square wall2"));
 }
 
 if((selGridInt == 1) && showInventory == true){
 prefabObject = (Resources.Load("wooden plank2"));
 }
 
 if((selGridInt == 2) && showInventory == true){
 prefabObject = (Resources.Load("rectangular wall"));
 }
 
 if((selGridInt == 3) && showInventory == true){
 prefabObject = (Resources.Load("ladder"));
 }
 
 if((selGridInt == 4) && showInventory == true){
 prefabObject = (Resources.Load("door"));
 }
 
 if((selGridInt == 5) && showInventory == true){
 prefabObject = (Resources.Load("floor"));
 }
 
 if((selGridInt == 6) && showInventory == true){
 prefabObject = (Resources.Load("window frame"));
 }
 
 if((selGridInt == 7) && showInventory == true){
 prefabObject = (Resources.Load("wall light"));
 }
 
 if((selGridInt == 8) && showInventory == true){
 prefabObject = (Resources.Load("fence"));
 }
 
 if((selGridInt == 9) && showInventory == true){
 prefabObject = (Resources.Load("glass wall"));
 }
 
 if((selGridInt == 10) && showInventory == true){
 prefabObject = (Resources.Load("column"));
 }
 
 if((selGridInt == 11) && showInventory == true){
 prefabObject = (Resources.Load("half wall up"));
 }
 
 if((selGridInt == 12) && showInventory == true){
 prefabObject = (Resources.Load("chair"));
 }
 
 if((selGridInt == 13) && showInventory == true){
 prefabObject = (Resources.Load("table"));
 }
 
 if((selGridInt == 14) && showInventory == true){
 prefabObject = (Resources.Load("half wall down"));
 }
 
 if((selGridInt == 15) && showInventory == true){
 prefabObject = (Resources.Load("platform"));
 }
 
 if((selGridInt == 16) && showInventory == true){
 prefabObject = (Resources.Load("cone"));
 }
 
 if((selGridInt == 17) && showInventory == true){
 prefabObject = (Resources.Load("glass floor roof"));
 }
 
 if((selGridInt == 18) && showInventory == true){
 prefabObject = (Resources.Load("???"));
 }
 
 if((selGridInt == 19) && showInventory == true){
 prefabObject = (Resources.Load("???"));
 }
 
 if((selGridInt == 20) && showInventory == true){
 prefabObject = (Resources.Load("???"));
 }
 
 if((selGridInt == 21) && showInventory == true){
 prefabObject = (Resources.Load("lamp"));
 }
 
 if((selGridInt == 22) && showInventory == true){
 prefabObject = (Resources.Load("plant"));
 }
 
 if((selGridInt == 23) && showInventory == true){
 prefabObject = (Resources.Load("big ass block"));
 }
 }
 }
 }