To cite gapfill in publications use:

F. Gerber, R. de Jong, M. E. Schaepman, G. Schaepman-Strub, and R. Furrer (2018) in IEEE Transactions on Geoscience and Remote Sensing, pp. 1-13, 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},
  }