Go to File > New Project > Ruby > Ruby Project with Existing Sources and choose a Ruby (not rails) existing project.

Get a little message saying "Looks like a Rails directory - Use a Rails project instead"

But it's not a Rails project - the Rails layout is pretty common for Ruby projects (although I'm not sure exactly what
NetBeans is looking for, or why it cares), and this seems like a strange thing to say.  If it's not a Rails project,
seems like I shouldn't claim to NetBeans that it's a Rails project.

And sure enough, when I go back, and try to set it up as a rails project, netbeans says "Not a Ruby on Rails application
directory."