For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/electronics/openSUSE_Tumbleweed/electronics.repo
zypper refresh
zypper install gnucap
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/electronics/15.6/electronics.repo
zypper refresh
zypper install gnucap
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/electronics/15.5/electronics.repo
zypper refresh
zypper install gnucap