You can use Bundler to isolate your gems locally, or you can use rbenv-gemset:

https://github.com/jamis/rbenv-gemset

For that, I use Bundler and always install gems to vendor/bundle for maximum isolation and easy cleanup.

:~ which bi

bi: aliased to bundle install --path vendor/bundle

You can tell Bundler to install to a relative path by default by putting the following in ~/.bundle/config:

  ---
  BUNDLE_PATH: vendor/bundle

In that way, you need only run the standard "bundle" or "bundle install" instead of setting up an alias.

(You should only have to run this the first time you bundle.)

but does this not increase the size of your source control repository?

No, just add the directory to .gitignore.