For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Gankov/15.6/home:Gankov.repo
zypper refresh
zypper install gtk-sharp2-gapi
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Gankov/15.5/home:Gankov.repo
zypper refresh
zypper install gtk-sharp2-gapi
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Gankov/15.4/home:Gankov.repo
zypper refresh
zypper install gtk-sharp2-gapi