I wouldn't call that a hidden gem, neither is RBS. A hidden gem would be Typeprof (https://rubygems.org/gems/typeprof https://rubygems.org/gems/typeprof) which

I wouldn't call that a hidden gem, neither is RBS. A hidden gem would be Typeprof (https://rubygems.org/gems/typeprof https://rubygems.org/gems/typeprof) which creates .rbs files from your ruby code. Very good for lazy people.