To cite gapfill in publications use:

Gerber F, de Jong R, Schaepman ME, Schaepman-Strub G, Furrer R (2018). “Predicting Missing Values in Spatio-Temporal Remote Sensing Data.” IEEE Transactions on Geoscience and Remote Sensing, 1–13. doi:10.1109/TGRS.2017.2785240, https://doi.org/10.1109/TGRS.2017.2785240.

Corresponding BibTeX entry:

  @Article{,
    title = {Predicting Missing Values in Spatio-Temporal Remote
      Sensing Data},
    author = {Florian Gerber and Rogier {de Jong} and Micheal E.
      Schaepman and Gabriela Schaepman-Strub and Reinhard Furrer},
    journal = {IEEE Transactions on Geoscience and Remote Sensing},
    year = {2018},
    pages = {1--13},
    url = {https://doi.org/10.1109/TGRS.2017.2785240},
    doi = {10.1109/TGRS.2017.2785240},
  }