Please cite GPBayes including its version number as

  Ma P (2023). “GPBayes: Tools for Gaussian Process Modeling in
  Uncertainty Quantification.” R package version 0.1.0-6,
  <https://CRAN.R-project.org/package=GPBayes>.

A BibTeX entry for LaTeX users is

  @Misc{,
    title = {GPBayes: Tools for Gaussian Process Modeling in Uncertainty Quantification},
    author = {Pulong Ma},
    note = {R package version 0.1.0-6},
    organization = {Iowa State University},
    address = {Ames, IA, USA},
    year = {2023},
    url = {https://CRAN.R-project.org/package=GPBayes},
  }