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

class example(MonoBehaviour):

    def OnGUI():
        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()