libgtkmm-2_4-1 from SUSE:SLE-15:Update project
Select Your Operating System
Add repository and install manually
For pool run the following as root:
zypper addrepo https://download.opensuse.org/repositories/SUSE:SLE-15:Update/pool/SUSE:SLE-15:Update.repo zypper refresh zypper install libgtkmm-2_4-1