In our setup, we have a class that does the heavy lifting. The file where that class lives ends with :

    if __name__ == '__main__':
        fire.Fire(JobManager)

which turns it into an instant CLI.

Then we have a separate gui.py file, which contains the Gooey code. This includes the class mentioned above, and builds a minimalistic GUI around it. This does need some argparse-based code, but in the GUI we only expose a small subset of the full power, so we only need a little bit of argparse-code.

P.S. I'm not certain this is the best way to approach things, but it's the way we use it, and we're happy with it so far.