This gem is almost a decade old (although looks like it's been updated to work with fibers?). There's a bunch of gems that allow for asynchronous/concurrent and/or parallel execution for Ruby. It looks like a nice enough gem but it's not particularly novel. Seems like it's just trying to ride JS' popularity with the name. Ruby threads also allow for non-blocking operations.

Edit - this isn't some new paradigm for Ruby. It's a gem... A bunch of others ("Parallel", "Concurrent-Ruby", "Eventmachine", etc...) have been around forever and do similar things.

Edit2 - https://rubygems.org/gems/async