It's a piece of cake to create a gemfile RPM package using the gem2rpm tool, but Fedora Package Guidelines for Ruby compliance is not that straightforward. Let me explain basic issues a packager will be facing to.
The fist part is a duck soup. Really. Set the gem2rpm against downloaded gem file. Most likely you'll need to redirect it's standard output to a file. That's your starting point spec file. So far, so good, isn't it?
Checkout the upstream license as the gem2rpm puts "GPLv2 or Ruby" constrant there.
This is not an universal blueprint. It only works for gems without native libraries or special artifacts. It has been prepared with Fedora 14 guidelines. Please note the guidelines can change in the future.