A local Gemfile where you can add gems that you don't want to add to the repository

Copy Gemfile.local into project root and add the following to your .gitignore:

/Gemfile.local
/Gemfile.local.lock

Also, you might want to duplicate your project's Gemfile.lock into Gemfile.local.lock to make sure gems are locked under the same versions. In your terminal run:

cp Gemfile.lock Gemfile.local.lock

To use Gemfile.local run:

bundle --gemfile Gemfile.local

Note: This should store a config in the .bundle/config file and then read from that, however I couldn't get this to work for the BUNDLE_GEMFILE config no matter what I tried. There is a bug request to add this as standard functionality to Bundler, however it appears there is nobody around to work on it: rubygems/bundler#183 (comment)