mkvtoolnix-gui from home:alois:leap155 project
Select Your Operating System
Add repository and install manually
For 15.5 Update run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:alois:leap155/15.5_Update/home:alois:leap155.repo zypper refresh zypper install mkvtoolnix-gui