To cite gremlin in publications use:

  Matthew E. Wolak (2020). gremlin: R package for mixed-effects model
  REML incorporating Generalized Inverses. Zenodo.
  doi.org/10.5281/zenodo.1476565

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{gremlin}: {R} package for mixed-effects model REML
                 incorporating Generalized Inverses},
    author = {Matthew E. Wolak},
    journal = {Zenodo},
    year = {2020},
    url = {https://doi.org/10.5281/zenodo.3908463},
  }