We've added a basic Ruby gem manager functionality in the latest EAP build.
It allows you to browse local gems inside a selected Ruby SDK and choose the ones you need in the project including the dependent ones. For Rails projects, the latest found version of Rails is auto-added.
That's almost all that we have now. But this is far from all we plan.
So, here is what's coming soon:
Browse remote gem repositories and install gems. rubyforge.org is the default, of course.
Want a new Rails project but Ruby on Rails is not there? Hardly possible, but in case it happens, the needed gems will