I do not understand what you mean by "the prior for 95% credible intervals". I do think you can determine the 95% credible interval from the values returned by your two functions. Take a look at the plot below You might also confirm your understanding of the "q" functions by inspecting the values returned by the qnorm() function since you may be more familiar with the standard normal distribution
qgamma(0.025, 10, 1)
#> [1] 4.795389
qgamma(0.975, 10, 1)
#> [1] 17.0848
plot(seq(0, 25, 0.1), dgamma(seq(0,25,0.1), 10,1), type = "l")
lines(c(4.795, 17.0848), c(0,0), col = "red", lwd = 2)

Created on 2020-02-11 by the reprex package (v0.2.1)