If you want to create a directly-executable cross-platform GUI app using Python first you will need some GUI library with Python bindings and then some program that will convert your python scripts into standalone executables.
Below is the name of cross-platform GUI libraries with Python bindings (Windows, Linux, Mac):-
Tkinter - You can use Tkinter and it is based on Tk GUI toolkit (de-facto standard GUI library for python, free for commercial projects)
WxPython - Another option you have is to use WxPython, based on WxWidgets (popular, free for commercial projects)
To know more about this you can have a look at the following video:-