For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:osmocom:nightly/openSUSE_Tumbleweed/home:mnhauke:osmocom:nightly.repo
zypper refresh
zypper install gapk
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:osmocom:nightly/15.5/home:mnhauke:osmocom:nightly.repo
zypper refresh
zypper install gapk
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:osmocom:nightly/15.4/home:mnhauke:osmocom:nightly.repo
zypper refresh
zypper install gapk