問題ない。http://www.fastpic.jp/images/102/8240119530.png

public class SelectionGrid : MonoBehaviour {
 public Texture[] textures = new Texture[4];
 public int selected = 2;
 
 void OnGUI () {
  selected = GUILayout.SelectionGrid (selected, textures, 2);
  Debug.Log ("Selected = " + selected);
 }
}