You've got at least these two options.
First one, use
apt-file for packages you haven't installed yet.
$ sudo apt-get install apt-file
$ sudo apt-file update
$ apt-file list usb-creator-gtk
Second one, use the packages.ubuntu.com website to locate the package, then all the way down on the page in the "Download" section, you'll find a link
[list of files]. Example for your package in Bionic (18.04) is here.
Either way, please remember that packages might install more files in their install scripts!