bundle install --without with private git repository

I have a Gemfile which contains this group referencing a private repo that I don't have access to:

group :dpla_branding do  gem 'dpla_frontend_assets', git: 'git@github.com:dpla/frontend-assets.git'end

and when I try to do:

gem install --without dpla_branding

I get an error saying the Git repository is not accessible. How can I get Bundler to not even try to access the repository?

If there's a way to change the Gemfile so that it works for both cases, I can submit a pull request to the upstream, but otherwise I'd prefer to avoid having developers edit the file.