For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:brassh:branches:science/openSUSE_Tumbleweed/home:brassh:branches:science.repo
zypper refresh
zypper install gmsh
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:brassh:branches:science/15.6/home:brassh:branches:science.repo
zypper refresh
zypper install gmsh
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:brassh:branches:science/15.5/home:brassh:branches:science.repo
zypper refresh
zypper install gmsh