Detect click on the outside of GUI.

ClickDetector.js

var guiIsHot : boolean;

function Update() {
	if (Input.GetMouseButton(0) && GUIUtility.hotControl != 0) {
		guiIsHot = true;
	}
	if (Input.GetMouseButtonUp(0)) {
		if (!guiIsHot) {
			Debug.Log("Hooray!");
		}
		guiIsHot = false;
	}
}

function OnGUI() {
	GUILayout.Button("button1");
	GUILayout.Button("button2");
}