You can do the following in your Gemfile, but then you'd have to switch the Gemfile between development and production, which is not recommended:

# Gemfile 
gem "foo", path: "/path/to/foo" 

Alternatively, you can tell Bundler to use a local gem in your current environment. This is preferred as the Gemfile still points to the production version of the gem. Run this in shell:

 bundle config set local.GEM_NAME /path/to/local/git/repository

Note: you have to set the branch of the gem, in Gemfile, for this to work.

You can see if it has worked: cat ./.bundle/config

There is also a user-wide setting that you can manually edit: cat ~/.bundle/config