How to build the package for a new release

On the master branch, add the last stable version with a release tag.

git tag -s v<VERSION> -m 'New release'
git push --tags

Checkout the branch debian, merge the master branch.

git checkout debian
git merge master --no-ff
dch -v <VERSION>-1
gbp buildpackage --git-debian-branch=debian --git-upstream-tree=master --git-export-dir=/tmp/build-area --git-ignore-new

If the build is OK, you can now build the final package.

dch -D stretch -r
gbp buildpackage --git-debian-branch=debian --git-upstream-tree=master --git-export-dir=/tmp/build-area --git-tag --git-sign --git-keyid=<KEY>