Hi David, thanks for sharing your question.
In the Ruby world, RubyGems (a.k.a. gems) are the predominant method of packaging and distributing code for use by other developers.
Due to the details of how the Ruby runtime loads gems from the file system, it’s not sufficient to pack up the content of the lib and bin directories from a repo like sensu-plugins/sensu-plugins-kubernetes to create a working asset.
The recommended way to create an asset for projects like sensu-plugins-kubernetes is to follow the steps documented in Contributing Assets for Existing Ruby Sensu Plugins
If you’d like take a shot at this yourself, you might want to look at pull requests from similar plugin collections which have already been packaged as assets to guide you. Examples include sensu-plugins-elasticsearch and sensu-plugins-disk-checks.
I hope this helps to put you on the right path. I’ve also opened https://github.com/sensu/sensu-docs/issues/1648 to track the need for these details to be noted in our asset reference documentation.