To cite gems in publications use:

Blaser N, Salazar Vizcaya L, Estill J, Zahnd C, Kalesan B, Egger M, Keiser O, Gsponer T (2015). “gems: An R Package for Simulating from Disease Progression Models.” Journal of Statistical Software, 64(10), 1–22. 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/},
  }