To cite tgp in publications use:

Robert B. Gramacy (2007). tgp: An R Package for Bayesian Nonstationary, Semiparametric Nonlinear Regression and Design by Treed Gaussian Process Models. Journal of Statistical Software, 19(9), 1-46. URL http://www.jstatsoft.org/v19/i09/.
Robert B. Gramacy, Matthew Taddy (2010). Categorical Inputs, Sensitivity Analysis, Optimization and Importance Tempering with tgp Version 2, an R Package for Treed Gaussian Process Models. Journal of Statistical Software, 33(6), 1-48. URL http://www.jstatsoft.org/v33/i06/.

Corresponding BibTeX entries:

  @Article{,
    title = {{tgp}: An {R} Package for Bayesian Nonstationary,
      Semiparametric Nonlinear Regression and Design by Treed Gaussian
      Process Models},
    author = {Robert B. Gramacy},
    journal = {Journal of Statistical Software},
    year = {2007},
    volume = {19},
    number = {9},
    pages = {1--46},
    url = {http://www.jstatsoft.org/v19/i09/},
  }
  @Article{,
    title = {Categorical Inputs, Sensitivity Analysis, Optimization and
      Importance Tempering with {tgp} Version 2, an {R} Package for
      Treed Gaussian Process Models},
    author = {Robert B. Gramacy and Matthew Taddy},
    journal = {Journal of Statistical Software},
    year = {2010},
    volume = {33},
    number = {6},
    pages = {1--48},
    url = {http://www.jstatsoft.org/v33/i06/},
  }