What I typically do in this kind of situation is to add a bunch of print() statements to my code to see if it's running. Add a print("hello") or some such to
on_input() in your gui script file. You should see the print from the init() function and if you have set up the input correctly you should also see repeated calls to on_input() when you move the mouse, click etc.
Once you have done this initial sanity check you need to start digging deeper: Do you actually evaluate the clicks correctly and does the appropriate code run when you click on the buttons of your menu?