To cite gumbel in publications use:

C. Dutang (2015), gumbel: package for Gumbel copula.

Corresponding BibTeX entry:

  @Manual{,
    title = {gumbel: package for Gumbel copula},
    author = {Christophe Dutang},
    year = {2015},
    note = {R package version 1.10-1},
  }