Taking another look, there is a deeper issue in addition to the Gatekeeper quarantine: the extension's code is attempting to load a Ruby 2.0.0 library from the Frameworks distributed with SketchUp, but SketchUp 2018 does not contain such a library, it has Ruby 2.2.0. So even if you figure out the quarantine thing, this extension isn't going to work under SU 2018!