https://phili.pe/posts/loading-additional-ruby-gems-in-development/
A Bundler plugin that installs additional gems listed in a separate Gemfile.
This is especially useful when you always use certain gems in development but don’t want to add it to the project’s Gemfile.
Blog post:
Run the following inside a folder that does not have a Gemfile in it:
$ bundle plugin install bundler-private_install
After running bundle install it will trigger another bundle install for any other Gemfile that contains eval_gemfile.
The assumption is that this other Gemfile has an eval_gemfile 'Gemfile', i.e. a dependency on the main Gemfile.
As to why you’d want to do this, see the blog post linked above.
The gem is available as open source under the terms of the MIT License.