To cite gumbel in publications use:

Dutang C (2018). gumbel: package for Gumbel copula. R package version 1.10-2.

Corresponding BibTeX entry:

  @Manual{,
    title = {gumbel: package for Gumbel copula},
    author = {Christophe Dutang},
    year = {2018},
    note = {R package version 1.10-2},
  }