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