To cite OptGS in publications use:

Wason JMS (2015). “OptGS: An R Package for Finding Near-Optimal Group-Sequential Designs.” Journal of Statistical Software, 66(2), 1–13. http://www.jstatsoft.org/v66/i02/.

Corresponding BibTeX entry:

  @Article{,
    title = {{OptGS}: An {R} Package for Finding Near-Optimal
      Group-Sequential Designs},
    author = {James M. S. Wason},
    journal = {Journal of Statistical Software},
    year = {2015},
    volume = {66},
    number = {2},
    pages = {1--13},
    url = {http://www.jstatsoft.org/v66/i02/},
  }