Consider following code:
----->8----
/* a.c */
#include <complex.h>
#include <stdio.h>
#include <gsl/gsl_complex.h>
int main(void)
{
    gsl_complex z;
    GSL_SET_COMPLEX(&z, -0.0, 0.0);
    printf("(%lf%+lf)\n", GSL_REAL(z), GSL_IMAG(z));
    return 0;
}
--->8------

$ gcc a.c -std=c11 -lgsl && ./a.out  # expected (-0.000000+0.000000)
(0.000000+0.000000)

(GSL compiled by: CC="gcc -std=c11" ./configure -q && make -s)

Similar wrong results could be reproduced for other special components,
i.e. NAN's or INFINITY's.  Issue should be fixed by using macro CMPLX, available in C11+.

Patch attached.