Using version 2.6 of GSL from ftp://ftp.gnu.org/gnu/gsl/gsl-2.6.tar.gz

OS is Ubuntu 18.04.2 LTS

Hardware is a Dell workstation with a 3.60 GHz Intel i7-4790 and 32 GB
of RAM

Compiler is gcc 7.4.0 with the -w option

Though the documentation for gsl_sf_gamma_inc_Q states that the first
element must be strictly greater than zero, the function returns a
zero when the first argument is equal to 0. The error handler is
invoked when the first argument is negative though. The libraries
SciPy and JMAT exhibit the same behavior so it seems as though this is
expected; perhaps the documentation should be updated then.

#include <gsl/gsl_sf.h>
#include <stdio.h>
#include <gsl/gsl_math.h>
#include <float.h>

int main (void){
        double out;
        out = gsl_sf_gamma_inc_Q(0, 2.2);

        printf("%.*f\n", DBL_DIG-1, out);
        printf("%.21e\n", out);
        return 0;
}