Nested Selection Grid?

The Editor GUI has a Foldout control; you'd have to replicate it in-game with a clickable icon+label.

Expandable GUI is easiest to accomplish with GUILayout:

 [pseudocode]
 
 GUILayout.BeginVertical();
 foreach button {
   button.open = GUIUtils.Foldout(button.name);
   if ( button.open ) {
     GUIUtils.BeginIndent(); // see below snippet
     // display contents of the button
     GUIUtils.EndIndent();
   }
 } 
 GUILayout.EndVertical();

I actually wrote a fully-clickable foldout control because I hate clicking the triangle:

(This is from my GUIUtils.cs file, comment out the (FontStyle)1 bit if you don't want bold text)

     /* These functions begin and end a horizontal indentation */
     public static void BeginIndent(string style) { GUILayout.BeginHorizontal(); GUILayout.Space(20); GUILayout.BeginVertical(style); }
     public static void BeginIndent() { BeginIndent(""); }
     public static void EndIndent() { GUILayout.EndVertical(); GUILayout.EndHorizontal(); }

 /////////////////////////////////////////////////////////////////////////////
     /*
     * This section makes an EditorGUILayout.Foldout, except you can actually click anywhere on the word to open it
     * instead of having to click on the teeny-tiny triangle.
     * 
     * ... what the bananas, Unity
     */ 
     private static GUIStyle openFoldoutStyle;
     private static GUIStyle closedFoldoutStyle;
     private static bool initted;
     
     private static void Init() {
         openFoldoutStyle = new GUIStyle( GUI.skin.FindStyle("Foldout") );
         openFoldoutStyle.fontStyle = (FontStyle)1;
         openFoldoutStyle.stretchHeight = true;
         closedFoldoutStyle = new GUIStyle( openFoldoutStyle );
         openFoldoutStyle.normal = openFoldoutStyle.onNormal;
         openFoldoutStyle.active = openFoldoutStyle.onActive;
         initted = true;
     }
     
     public static bool Foldout(bool open, string text ) { return Foldout(open, new GUIContent(text)); }
     public static bool Foldout(bool open, GUIContent text ) {
         if ( !initted ) Init();
         
         if ( open ) { 
             if ( GUILayout.Button( text, openFoldoutStyle, GUILayout.Height(20) ) ) {
                 GUI.FocusControl ("");
                 GUI.changed = false; // force change-checking group to take notice
                 GUI.changed = true;
                 return false;
             }
         } else {
             if ( GUILayout.Button( text, closedFoldoutStyle, GUILayout.Height(20) ) ) { 
                 GUI.FocusControl ("");
                 GUI.changed = false; // force change-checking to take notice
                 GUI.changed = true;
                 return true;
             }
         }
         return open;
     }