To cite greta in publications use:

  Nick Golding (2019). greta: simple and scalable statistical modelling
  in R. Journal of Open Source Software, 4(40), 1601. URL
  http://dx.doi.org/10.21105/joss.01601.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{greta}: simple and scalable statistical modelling in R},
    author = {Nick Golding},
    journal = {Journal of Open Source Software},
    year = {2019},
    volume = {4},
    number = {40},
    pages = {1601},
    url = {http://dx.doi.org/10.21105/joss.01601},
    doi = {10.21105/joss.01601},
  }