git-debrebase does not do any builds.  You use your existing favourite
build tools for that.  Nor does git-debrebase do anything to do with
actual source packages (that is, .dsc files).  Typically, other tools
will automatically convert your git branch into an identical source
package, when you upload.

git-debrebase does not deal with orig tarballs, either.  It is a
purely git tool, so it consumes upstream source code in gitish form.
Ideally, you base your work on upstream git tags, and an equivalent
orig tarball (or one made with git-deborig).  Otherwise, you can
import an upstream tarball with a tool gbp-import-orig.

git-debrebase concentrates on managing the Debian packaging, and the
Debian delta queue - 

[usp slide]





, and the Debian packaging.  - the changes you make
in bb



(When you are uploading a new upstream to Debian,
you'll use tools like git-deborig or git-archive to generate a .orig
corresponding to the upstream source.)