To cite gsbDesign in publications use:

  Florian Gerber, Thomas Gsponer (2016). gsbDesign: An R Package for
  Evaluating the Operating Characteristics of a Group Sequential
  Bayesian Design. Journal of Statistical Software, 69(11), 1-23.
  doi:10.18637/jss.v069.i11

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{gsbDesign}: An {R} Package for Evaluating the Operating Characteristics of a Group Sequential {B}ayesian Design},
    author = {Florian Gerber and Thomas Gsponer},
    journal = {Journal of Statistical Software},
    year = {2016},
    volume = {69},
    number = {11},
    pages = {1--23},
    doi = {10.18637/jss.v069.i11},
  }

Thomas Gsponer, Florian Gerber, Bjorn Bornkamp, David Ohlssen, Marc
Vandemeulebroecke, Heinz Schmidli (2014). A Practical Guide to Bayesian
Group Sequential Designs. Pharmaceutical Statistics, 13(1), 71-80.
doi:10.1002/pst.1593

A BibTeX entry for LaTeX users is

  @Article{,
    title = {A Practical Guide to {B}ayesian Group Sequential Designs},
    author = {Thomas Gsponer and Florian Gerber and Bjorn Bornkamp and David Ohlssen and Marc Vandemeulebroecke and Heinz Schmidli},
    journal = {Pharmaceutical Statistics},
    year = {2014},
    volume = {13},
    number = {1},
    pages = {71--80},
    doi = {10.1002/pst.1593},
    issn = {1539-1612},
  }