Toggling a basic graphical GUI overlay using the escape key.

I did something like:

 bool menu=false;
 
 void Update()
 {
     if(Input.GetKeyDown(KeyCode.Escape))
     {
         if(menu==true)
         {
             menu=false
             Time.timeScale=0;
         }
         else if(menuLevel==false)
             {
                menu=true;
                Time.timeScale=1;
             }
 }

And then included if(menu) in my OnGUI(). This will also pause/unpause (most of) the game.

Create a GUITexture :(Menu -> GameObject -> Create other-> GUITexture)

This is an object in your gamescene, the transform' position of this object is tricky:

transform.position.x or y has to be between 0 or 1. For x, 0 is the most left side of the screen, 1 is the most right side of the screen, and for y, 0 is the most down side, and 1 most top, no matter the size of the screen!

GUITexture.pixelInset is something total different, this gives you the ability to reposition/size the texture from the relative point you've chosen with the transform.position positions (some kind of margin or padding)

So when setting the transform.position.x/y to 0, it should be position at the down-left corner of your screen (yes x:0, y:0 in GUI textures is downleft) and now we can scale the texture to the screen pixel size:

Oh yea, the transform.position.z is the index overlay, so other GUITextures with higher/lower values will be behind/above this texture.

So to actually turn this magic into pixels, attach a GUITexture into the scripts inspector, and almost forgot: attach a texture in the GUITexture :)

GL

     public GUITexture textureOverlay;
     public int overlayDepth = 1;
     private bool menuEnabled = false;
 
     void Start()
     {
         //on start, make texture same size as the screen.
         textureOverlay.transform.position = new Vector3(0, 0, overlayDepth);
         textureOverlay.pixelInset = new Rect(0, 0, Screen.width, Screen.height);
         textureOverlay.enabled = false;
     }
     void Update()
     {
        //toggle menu on escape key up
        if(Input.GetKeyUp(KeyCode.Escape))
        {
            menuEnabled = !menuEnabled;
        }
        //show or hide texture
        switch (menuEnabled)
        { 
            case true:
                if(!textureOverlay.enabled)
                     textureOverlay.enabled = true;
                break;
            case false:
                if(textureOverlay.enabled)
                     textureOverlay.enabled = false;
                break;
        }
     }

I agree with RadWayne. If you're using Unity's character controller, it isn't paused by the game so you can enable/disable it (vise versa too) with this (which will basically pause the game):

 if (menu) {
 GetComponent(MouseLook).enabled = false;
 Time.timeScale = 0;
 }
 
 else {
 if (!menu) {
 GetComponent(MouseLook).enabled = true;
 Time.timeScale = 1;
 }
 }