Is there a GUI for packaging and uploading a file to the Ubuntu repository?
No command-line stuff with the setup.
Yes, there is GUI for packaging. You can find some in this old answer: Graphical tool for creating Deb packages
But to upload to Launchpad, no.
Anyway, it's easy and you can do with a simple command.
The only caveat: first time only, you need to create SSH keys, register at Launchpad, etc: http://packaging.ubuntu.com/html/getting-set-up.html
Then, after create your Launchpad repo, no GUI is required, really simple:
dput ppa:<launchpad_user>/<repo> <package>_source.changes