How to remove a GUI element when button is pressed C#

Unity uses an immediate-mode GUI system. The GUI is drawn every frame. If you want to hide an element, don't draw it. for example:

 bool hasBeenPressed = false;
 void OnGUI()
 {
     if (!hasBeenPressed)
     {
         if(GUI.button(new rect(0,0,25, 25), "Button that needs to be removed"))
         {
             hasBeenPressed = true;
         }
     }
 }

If you want to hide the whole GUI in this script (and of course the script doesn't have other functions), you can also disable the script:

 void OnGUI()
 {
     if(GUI.button(new rect(0,0,25, 25), "Button that needs to be removed"))
     {
         enabled = false;
     }
 }