To cite tgp in publications use:

Gramacy RB (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. doi:10.18637/jss.v019.i09, https://www.jstatsoft.org/v19/i09/.

Gramacy RB, Taddy M (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. doi:10.18637/jss.v033.i06, https://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 = {https://www.jstatsoft.org/v19/i09/},
    doi = {10.18637/jss.v019.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 = {https://www.jstatsoft.org/v33/i06/},
    doi = {10.18637/jss.v033.i06},
  }