Well, buttons can have the value 0 but, again assuming I'm looking at the right tutorial, object 0 on your GUI would be the label. So yes, you probably want button 1 to quit, button 2 to keep on playing.
QuitGame (1); will ask you to confirm quitting (i.e. bring up the built-in Quit GUI, after your custom one), while QuitGame (0); will just quit when you press the button.
QuitGame (1); will ask you to confirm quitting (i.e. bring up the built-in Quit GUI, after your custom one), while QuitGame (0); will just quit when you press the button.