The gpgcheck option, found in the main section of the /etc/zypp/zypp.conf and individual /etc/zypp/repos.d/*.repo files determines if an RPM package's signature is checked prior to its installation. Rationale: It is important to ensure that an RPM's package signature is always checked prior to installation to ensure that the software is obtained from a trusted source.
Solution
Edit /etc/zypp/zypp.conf and set 'gpgcheck=1' in the [main] section. Edit any failing files in /etc/zypp/repos.d/*.repo and set all instances of gpgcheck to 1.