Charlotte, thanks for using GSL!
I agree with those who suggest using a gnu/linux distribution for all computing, and especially for scientific software development. But if you are stuck with macos then there are a few packaging systems that offer GSL. The two most used proper packaging systems on macs are homebrew, where you will find: https://formulae.brew.sh/formula/gsl and macports, where you will find: https://ports.macports.org/port/gsl/summary But also note that these packaging systems are barely needed for gsl because we have designed it to have minimal requirements. You should be able to download the source tarball from https://www.gnu.org/software/gsl/ and follow the instructions in the INSTALL file: it's the usual "./configure" and "make -j n_cores install" procedure. If your own program still does not find the GSL header files then you will need to get comfortable with the compiler -I flag to specify the directory in which you installed GSL's headers, and the linker's -L flag to tell it where the libraries were installed. Since details of compiling might be noise on this list, feel free to write individual email once it gets down to those details.
