To cite fgui in publications use:

Hoffmann TJ, Laird NM (2009). “fgui: A Method for Automatically Creating Graphical User Interfaces for Command-Line R Packages.” Journal of Statistical Software, 30(2), 1–14. http://www.jstatsoft.org/v30/i02/.

Corresponding BibTeX entry:

  @Article{,
    title = {{fgui}: A Method for Automatically Creating Graphical User
      Interfaces for Command-Line {R} Packages},
    author = {Thomas J. Hoffmann and Nan M. Laird},
    journal = {Journal of Statistical Software},
    year = {2009},
    volume = {30},
    number = {2},
    pages = {1--14},
    url = {http://www.jstatsoft.org/v30/i02/},
  }