To cite the 'gender' package, you may either cite the package directly
or cite the journal article which explains its method:

  Lincoln Mullen (2021). gender: Predict Gender from Names Using
  Historical Data. R package version 0.6.0.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {gender: Predict Gender from Names Using Historical Data},
    author = {Lincoln Mullen},
    year = {2021},
    note = {R package version 0.6.0},
    url = {https://github.com/lmullen/gender},
  }

For the journal article, please cite:

Cameron Blevins and Lincoln Mullen, "Jane, John ... Leslie? A
Historical Method for Algorithmic Gender Prediction," _Digital
Humanities Quarterly_ 9, no. 3 (2015):
<http://www.digitalhumanities.org/dhq/vol/9/3/000223/000223.html>.