To cite package 'RgoogleMaps' in publications use:

Loecher M, Ropkins K (2015). “RgoogleMaps and loa: Unleashing R Graphics Power on Map Tiles.” Journal of Statistical Software, 63(4), 1–18. http://www.jstatsoft.org/v63/i04/.

Corresponding BibTeX entry:

  @Article{,
    title = {{RgoogleMaps} and {loa}: Unleashing {R} Graphics Power on
      Map Tiles},
    author = {Markus Loecher and Karl Ropkins},
    journal = {Journal of Statistical Software},
    year = {2015},
    volume = {63},
    number = {4},
    pages = {1--18},
    url = {http://www.jstatsoft.org/v63/i04/},
  }