To cite gapfill in publications use:

F. Gerber, R. Furrer, G. Schaepman-Strub, R. de Jong, and M.E. Schaepman (2016). Predicting missing values in spatio-temporal satellite data. http://arxiv.org/abs/1605.01038

Corresponding BibTeX entry:

  @Article{,
    title = {Predicting missing values in spatio-temporal satellite
      data},
    author = {Florian Gerber and Reinhard Furrer and Gabriela
      Schaepman-Strub and Rogier {de Jong} and Micheal E. Schaepman},
    journal = {ArXiv e-prints},
    year = {2016},
    url = {http://arxiv.org/abs/1605.01038},
  }