Bayesian package

We are trying to obtain a predictive distribution in a Poisson model using a
non-informative prior distribution. We have found a gamma poisson as a
result of our analysis and we want to evaluate it in R. Is there any package
where this distribution and anothers are defined?

In any case, we have tried to programm the gammapoisson probability function
but we have found another problem: we must evaluate a gamma function with a
quite high value (gamma(284)). How can it be done?

Thank you very much!