To cite GAS in publications use:

Ardia D, Boudt K, Catania L (2019). “Generalized Autoregressive Score Models in R: The GAS Package.” Journal of Statistical Software, 88(6), 1–28. doi: 10.18637/jss.v088.i06.

Corresponding BibTeX entry:

  @Article{,
    title = {Generalized Autoregressive Score Models in {R}: The {GAS}
      Package},
    author = {David Ardia and Kris Boudt and Leopoldo Catania},
    journal = {Journal of Statistical Software},
    year = {2019},
    volume = {88},
    number = {6},
    pages = {1--28},
    doi = {10.18637/jss.v088.i06},
  }