A little while ago I wrote a post regarding how to work with Ctrlp and your Gem directory effectively. I've since found a way that I like a lot more. The one issue with the following approach is that it assumes that you are using a Gemfile.

To start, toss this in your vimrc:

" open gem dir in ctrlp
function! GemCtrlP(gem_name)
  let path = system('bundle list ' . a:gem_name . ' | tr -d "\n"')
  execute 'CtrlP ' . path
command! -bang -nargs=* -complete=file Gem call GemCtrlP(<q-args>)

Usage works like so: :Gem rails

The function! call defines the GemCtrlP function. The bang will overwrite the function if it is already defined (e.g. if you source your .vimrc). The command! call allows you to call :Gem my_gem in command mode. The function works by making a syscall to bundler to get the gem path. It then opens Ctrlp in that path.