Invalid parameter values will return an error detailing which parameter is problematic.
Details
The Gamma distribution with shape parameter \(\alpha\) and rate
parameter \(\beta\) has density:
$$f\left(x\right) = \frac{\beta^{\alpha}}{\Gamma(\alpha)} x^{\alpha - 1}%
\textrm{e}^{-\beta x}$$
for \(x \in \mathcal{R}^+\), \(\beta, \alpha > 0\).