To cite gdistance in publications use:

Jacob van Etten (2017). R Package gdistance: Distances and Routes on Geographical Grids. Journal of Statistical Software, 76(13), 1-21.<doi:10.18637/jss.v076.i13>

Corresponding BibTeX entry:

  @Article{,
    title = {{R} Package {gdistance}: Distances and Routes on
      Geographical Grids},
    author = {Jacob {van Etten}},
    journal = {Journal of Statistical Software},
    year = {2017},
    volume = {76},
    number = {13},
    pages = {1--21},
    doi = {10.18637/jss.v076.i13},
  }