with vim, NERDTree, ack, and gem edit: once you `gem edit` and the directory list opens, go to the directory listing (e.g. /Users/you/.rvm/gems/ruby-1.9.2-p136/gems/twitter-1.1.1/) and type "cd". This changes your current working directory to the gem root, which in turn allows you to `:Ack some_text` from the gem root which is very useful.