Sitting next to the developer of MacGap (we both work at Twitter), the first thing that I asked was "what about Fluid?" http://fluidapp.com/ -- and Alex's first retort was that this was open source, and free.
I'm more curious why he forked Phonegap-mac?
See the archived changelog here: http://www.myosxfreeware.com/fluid-04/
Specifically "FluidInstance.app: SSBs can now run local Ruby on Rails webapps."
There used to be a way to package up a RoR app inside the application contents and distribute that as your app. No need for an actual hosted website.
However - the feature was poorly documented and never really taken advantage of. Since they went 1.0, I believe it's also been removed.