Bundler already does this.

  # From a specific branch
  gem 'my_gem', git: 'https://github.com/user/my_gem.git', branch: 'development'

  # From a specific tag
  gem 'my_gem', git: 'https://github.com/user/my_gem.git', tag: 'v1.2.3'

  # From a specific commit (ref)
  gem 'my_gem', git: 'https://github.com/user/my_gem.git', ref: 'a1b2c3d4e5f6g7h8i9j0k1l2m3n4o5p6q7r8s9t0'

Yes, I know bundler does that. But I thought we were talking about urn instead of uri. Seems I was mistaken.