To cite gRain in publications use:

  Søren Højsgaard (2012). Graphical Independence Networks with the
  gRain Package for R. Journal of Statistical Software, 46(10), 1-26.
  URL https://www.jstatsoft.org/v46/i10/

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Graphical Independence Networks with the {gRain} Package for {R}},
    author = {S{\o}ren H{\o}jsgaard},
    journal = {Journal of Statistical Software},
    year = {2012},
    volume = {46},
    number = {10},
    pages = {1--26},
    doi = {10.18637/jss.v046.i10},
    url = {https://www.jstatsoft.org/v46/i10/},
  }