The gsl postinstall script runs /sbin/install-info /usr/info/gsl-ref.info.gz /usr/info/dir Unfortunately, it redirects errors to nowhere, and thus it (and everyone) fails to notice that actually there is no /usr/info/gsl-ref.info.gz file, and it's instead just /usr/info/gsl-ref.info. I'm not sure what the bug is: that the info files are not compressed, or that install-info is run on the wrong file. I leave that up to Red Hat to decide.
gsl is not included in RHL 6.1.