I renamed the file gsl.pc.in as gsl.pc and placed it in the pkg-config root directory

That procedure sounds a little suspicious ;-)

Normally, those .in files are templates that are filled in with the appropriate metainfo (like compile/link options, version, install paths, etc.) at the time the library is configured and built. In other words, the template itself is unlikely to contain any useful info for a pkg-config query...