How do I make a GUI menu that pops up?

Example:

 bool isOpen = false;
 
 if(Input.GetKeyDown("escape")){
     //Show the screen
     isOpen = true;
 }
 
 if(Input.GetKeyDown("escape")){
     if(isOpen)
        //Don't show the screen
 }

 public class ToggleGUI : MonoBehaviour {
    private bool on = false;
 
    void Update() {
       //Use one of these 3 ways to toggle your menu!

       //Type 1: Use the same key to on/off
       if( Input.GetKeyDown( KeyCode.Q ) ) {
          on = !on;
       }
 
       //Type 2: Use one key to turn on 
       //        another key to off 
       if( Input.GetKeyDown( KeyCode.Q ) ) {
          on = true;
       }
       else if( Input.GetKeyDown( KeyCode.E ) ) {
          on = false;
       }
 
       //Type 3: Hold a key to keep the menu open
       if( Input.GetKeyDown( KeyCode.Q ) ) {
          on = true;
       }
       else if( Input.GetKeyUp( KeyCode.Q ) ) {
          on = false;
       }
    }
 
    void OnGUI() {
       if( on ) {
          ...
       }
    }
 }

It is a bit of a overkill, but I wrote in 3 difference menu-toggle-by-key behaviours. Use whichever one you prefer.