gtk3 from home:MaxxedSUSE:Compiler-Tools-leap project
Select Your Operating System
Add repository and install manually
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:MaxxedSUSE:Compiler-Tools-leap/15.6/home:MaxxedSUSE:Compiler-Tools-leap.repo zypper refresh zypper install gtk3