For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:gns3/openSUSE_Tumbleweed/home:mnhauke:gns3.repo
zypper refresh
zypper install gns3-server
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:gns3/15.4/home:mnhauke:gns3.repo
zypper refresh
zypper install gns3-server
For 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:gns3/15.3/home:mnhauke:gns3.repo
zypper refresh
zypper install gns3-server