Update: Even easier than the method described below is to set your EDITOR environment variable to subl and use

bundle open devise

This command opens the ‘devise’ gem in Sublime Text 2. I use Bundler to manage gems:

bundle show devise | xargs sub

And as an aside, while researching the bundle open command, I also ran into

bundle console

which opens a Ruby IRB session with all the gems in your bundle loaded. Great if you need a console where you want your gems, but you don’t want to load the Rails app.