For this package, I have experimented on an alternative layout for the git repository, following the section entitled "When upstream uses Git" in the git-buildpackage manual [1].
Since the repository only contains the "debian/sid" and "pristine-tar" branches (no upstream or master), it does not play very nice with the defaults of gbp-clone and co.
I can definitely checkout the repository with the following command: gbp-clone git+ssh://git.debian.org/git/debian-med/irtk.git --upstream-branch=master --debian-branch=debian/sid
And build with: gbp buildpackage --git-upstream-tag='v%(version)s' --git-debian-branch=debian/sid
I quite like the workflow explained in the manual, where you just fetch the new upstream tag,
merge in a debian/<release> branch and use the --git-upstream-tag option to specify the upstream source. I'll get back to the usual workflow with "gbp import-orig" if that is a problem, it was really just by curiosity that I tried this.