what does " gbp:error: upstream/1.5.13 is not a valid treeish" mean?

The current tag/branch you are in, is not a Debian source tree, it doesn't contain the debian/ directory in its root. This is evident because you are using a "upstream/" branch, a name utilized to upload the pristine source tree to git repositories. Try using the branch stable, testing or unstable, or any branch that starts with Debian or a commit tagged using the Debian versioning scheme.

This can be caused by the tarball not being present in the parent directory. I get this (highly crypric) message even with a debian/ folder present.

My solution was to use uscan to get the watch file to download a fresh copy of the tarball

uscan --force-download