As someone who has written and sold a commercial app written in python and pyqt, I'd like to comment.
No, you cannot add a GUI without modifying your code.
How much it would have to be modified would depend on how it was written. If you plan on adding a GUI at some point, it's always good to structure your code with that thought in mind.
In the future though, if you are planning to develop more desktop apps, I would stay away from python altogether.
There is no good bundling method for python apps. They can be hard to distribute without further modification. Nothing ever goes smoothly when bundling a python app unless it's an extremely simple app.
I have been using angular + electron for desktop apps lately, and would highly recommend the combination. Things tend to go smoothly, and when they don't, there is an abundant amount of information on stack overflow concerning electron.
I'm looking forward to when flutter for desktop is finally stable for all platforms and has good plugin support. That will be a great day for desktop development.