If a gem gets pulled from rubygems.org, but you still have it locally (or can get hold of it somehow), you can move the gem into your vendor/gems directory.
Unpack the gem into vendor/gems
gem unpack <gem-name> --target vendor/gems
Edit the Gemfile to match
gem '<gem-name>', :path => 'vendor/gems/<gem-directory>'
If you get errors such as:
config.gem: Unpacked gem <gem-name> in vendor/gems has no specification file. Run 'rake gems:refresh_specs' to fix this.
But when you run rake gems:refresh_specs it fails :( Do this:
cd vendor/gems/<gem-name>
gem specification <gem-name> > .specification