Automagic GUI Scaling in Unity3D

Step 1. Tell you team that everything must fit within 1920x1080 or 1024x768 or whatever resolution you like!

Step 2. Surround all OnGUI method body code with methods BeginGUI and EndGUI from the below class.

Step 3. Scaling across all resolutions... problem solved!

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class GUISizer {

    static float  WIDTH = 1024;
    static float HEIGHT = 768;

    static List<Matrix4x4> stack = new List<Matrix4x4> ();

    static public void BeginGUI() {
        stack.Add (GUI.matrix);
        Matrix4x4 m = new Matrix4x4 ();
        var w = (float)Screen.width;
        var h = (float)Screen.height;
        var aspect = w / h;
        var scale = 1f;
        var offset = Vector3.zero;
        if(aspect < (WIDTH/HEIGHT)) { //screen is taller
            scale = (Screen.width/WIDTH);
            offset.y += (Screen.height-(HEIGHT*scale))*0.5f;

        } else { // screen is wider
            scale = (Screen.height/HEIGHT);
            offset.x += (Screen.width-(WIDTH*scale))*0.5f;
        }
        m.SetTRS(offset,Quaternion.identity,Vector3.one*scale);
        GUI.matrix *= m;
    }

    static public void EndGUI() {
        GUI.matrix = stack[stack.Count - 1];
        stack.RemoveAt (stack.Count - 1);
    }
    
}