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.