Our paper about GPstuff has been published in Journal of Machine Learning Research
http://jmlr.csail.mit.edu/papers/v14/vanhatalo13a.html
If you use GPstuff, please use the reference: Jarno Vanhatalo, Jaakko
Riihim�ki, Jouni Hartikainen, Pasi Jyl�nki, Ville Tolvanen, Aki Vehtari (2013).
GPstuff: Bayesian Modeling with Gaussian Processes. In Journal of Machine
Learning Research, 14:1175-1179.
The code is now also available at
http://mloss.org/software/view/451/