To cite GPfit in publications use:

Blake MacDonald, Pritam Ranjan, Hugh Chipman (2015). GPfit: An R Package for Fitting a Gaussian Process Model to Deterministic Simulator Outputs. Journal of Statistical Software, 64(12), 1-23. URL http://www.jstatsoft.org/v64/i12/.

Corresponding BibTeX entry:

  @Article{,
    title = {{GPfit}: An {R} Package for Fitting a Gaussian Process
      Model to Deterministic Simulator Outputs},
    author = {Blake MacDonald and Pritam Ranjan and Hugh Chipman},
    journal = {Journal of Statistical Software},
    year = {2015},
    volume = {64},
    number = {12},
    pages = {1--23},
    url = {http://www.jstatsoft.org/v64/i12/},
  }