That's because there isn't really a good "bundler equivalent" that I know of. That was one of the more frustrating parts of Python work for me. Virtualenv is kind of a hybrid between gem and bundler, and it works, but it's nowhere near as elegant.
The jobs are divided a bit differently but the combo of gem/rvm/bundler is matched quite nicely by pip and virtualenv (with virtualenvwrapper adding some nice conveniences)
This article covers some of the equivalent jobs using Ruby and Python:
