To cite gumbel in publications use:

C. Dutang (2008), gumbel: package for Gumbel copula.

Corresponding BibTeX entry:

  @Manual{,
    title = {gumbel: package for Gumbel copula},
    author = {Anne-Lise Caillat and Christophe Dutang and V'eronique
      Larrieu and Triet NGuyen},
    year = {2008},
    note = {R package version 1.01},
  }