For SLE 15 SP2 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:odassau/SLE_15_SP2/home:odassau.repo
zypper refresh
zypper install gsl
For SLE 12 SP5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:odassau/SLE_12_SP5/home:odassau.repo
zypper refresh
zypper install gsl
For SLE 12 SP4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:odassau/SLE_12_SP4/home:odassau.repo
zypper refresh
zypper install gsl