To cite gratis in publications use:

Kang Y, Li F, Hyndman RJ, O'Hara-Wild M, Zhao B (2020). gratis: GeneRAting TIme Series with diverse and controllable characteristics. R package version 0.2-1, https://CRAN.R-project.org/package=gratis.

Kang Y, Li F, Hyndman RJ (2020). “GRATIS: GeneRAting TIme Series with diverse and controllable characteristics.” Statistical Analysis and Data Mining, 13(4), 354–376. doi: 10.1002/sam.11461, https://onlinelibrary.wiley.com/doi/abs/10.1002/sam.11461.

Corresponding BibTeX entries:

  @Manual{,
    title = {{gratis}: GeneRAting TIme Series with diverse and
      controllable characteristics},
    author = {Yanfei Kang and Feng Li and Rob J Hyndman and Mitchell
      O'Hara-Wild and Bocong Zhao},
    year = {2020},
    note = {R package version 0.2-1},
    url = {https://CRAN.R-project.org/package=gratis},
  }
  @Article{,
    title = {{GRATIS}: GeneRAting TIme Series with diverse and
      controllable characteristics},
    author = {Yanfei Kang and Feng Li and Rob J Hyndman},
    journal = {Statistical Analysis and Data Mining},
    year = {2020},
    volume = {13},
    number = {4},
    pages = {354--376},
    url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/sam.11461},
    doi = {10.1002/sam.11461},
  }