To cite GFD in publications use:

Friedrich S, Konietschke F, Pauly M (2017). “GFD: An R Package for the Analysis of General Factorial Designs.” Journal of Statistical Software, Code Snippets, 79(1), 1–18. doi:10.18637/jss.v079.c01.

Corresponding BibTeX entry:

  @Article{,
    title = {{GFD}: An {R} Package for the Analysis of General
      Factorial Designs},
    author = {Sarah Friedrich and Frank Konietschke and Markus Pauly},
    journal = {Journal of Statistical Software, Code Snippets},
    year = {2017},
    volume = {79},
    number = {1},
    pages = {1--18},
    doi = {10.18637/jss.v079.c01},
  }