To cite gems in publications use:

Nello Blaser, Luisa Salazar Vizcaya, Janne Estill, Cindy Zahnd, Bindu Kalesan, Matthias Egger, Olivia Keiser, Thomas Gsponer (2015). gems: An R Package for Simulating from Disease Progression Models. Journal of Statistical Software, 64(10), 1-22. URL http://www.jstatsoft.org/v64/i10/.

Corresponding BibTeX entry:

  @Article{,
    title = {{gems}: An {R} Package for Simulating from Disease
      Progression Models},
    author = {Nello Blaser and Luisa {Salazar Vizcaya} and Janne Estill
      and Cindy Zahnd and Bindu Kalesan and Matthias Egger and Olivia
      Keiser and Thomas Gsponer},
    journal = {Journal of Statistical Software},
    year = {2015},
    volume = {64},
    number = {10},
    pages = {1--22},
    url = {http://www.jstatsoft.org/v64/i10/},
  }