about the percentiles of a Gamma distribution

[Stefano Sofia <stefano.sofia@usa.net>]

I need a trivial confirm: given a Gamma distribution with 

shape = alpha and rate = beta

I need to evaluate the p_th percentile. For the 10th percentile of a
Gamma(12.51, 1.41), for example, it would simply be

pgamma(10, 12.51, 1.41)

Correct?
Why pgamma(q, 12.51, 1.41) for q greater than 30 always gives as result 1?

thank you
Stefano