ManageIQ UI Classic is the UI that has been split out of the main manageiq repo.
Go to the directory where you have the clone of your 'manageiq' repo.
Next to your 'manageiq' repo create a clone of 'manageiq-ui-classic' repo:
$ git clone email@example.com:ManageIQ/manageiq-ui-classic.git
This way you will have your 'manageiq' and 'manageiq-ui-classic' side-by-side.
At this point you are ready and can start doing PRs against any of the 2 repos.
Or you can get more detailed instructions in ManageIQ/guides.
The gem is available as open source under the terms of the Apache License 2.0.