End a group.

	function OnGUI () {
		// Constrain all drawing to be within a 800x600 pixel area centered on the screen.
		GUI.BeginGroup (new Rect (Screen.width / 2 - 400, Screen.height / 2 - 300, 800, 600));
	
		// Draw a box in the new coordinate space defined by the BeginGroup.
		// Notice how (0,0) has now been moved on-screen
		GUI.Box (new Rect (0,0,800,600),
			"This box is now centered! - here you would put your main menu");
	
		// We need to match all BeginGroup calls with an EndGroup
		GUI.EndGroup ();
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnGUI() {
        GUI.BeginGroup(new Rect(Screen.width / 2 - 400, Screen.height / 2 - 300, 800, 600));
        GUI.Box(new Rect(0, 0, 800, 600), "This box is now centered! - here you would put your main menu");
        GUI.EndGroup();
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnGUI() as void:
		GUI.BeginGroup(Rect(((Screen.width / 2) - 400), ((Screen.height / 2) - 300), 800, 600))
		GUI.Box(Rect(0, 0, 800, 600), 'This box is now centered! - here you would put your main menu')
		GUI.EndGroup()