You would not need separate packages to do that (in fact, you can't do this with separate packages because dpkg will complain if two packages provide the same file).

  sudo update-alternatives --set python3 /usr/bin/python3-nogil
  sudo update-alternatives --set python /usr/bin/python3

The PEP actually states that the nogil version would also have env variable allowing to temporarily enable GIL. Although I guess in practice they might still build separate versions.