GUI.button and onClick Terrain

Well if you define your pause button Rect in start or awake, you can setup a mouse position and check if that Rect contains it. Then you can have a bool that determine whether or not you are allowed to fire your gun. Since you didn't post your code, I'll give you a basic, easy to understand example. Hope t$$anonymous$$s helps :)

 Rect pauseBtnRect;
 Vector2 mouse;
 bool canFire;
 
 void Awake()
 {
     pauseBtnRect = new Rect(0, 0, 128, 64);
 }
 
 void Update()
 {
     mouse = new Vector2(Input.mousePosition.x, Screen.height - Input.mousePosition.y);
 
     if(pauseBtnRect.Contains(mouse))
         canFire = false;
     else
         canFire = true;
 
     if(canFire)
     {
         if(Input.GetButton("Fire1"))
         //Your firing code
     }
 
 }
 
 void OnGUI()
 {
     if(GUI.Button(pauseBtnRect, "Pause"))
         //Your pause code.
 }