First, create a branch that holds the NMUs from the tip of your debian-branch (default is master) once:

  git branch nmu master

To import an NMU, change into the git repository and use gbp import-dsc:

  git checkout master
  gbp import-dsc --debian-branch=nmu /path/to/package_1.0-1nmu0.dsc

This will import the NMU onto the branched named nmu instead of the default master. This method can also be used to import "old" releases into the Git repository when migrating to Git from another VCS.