cms.gov is medicare back-end, not sure if this is an extension of that or part of the original
The Healthcare.gov Github repo does live under the CMS.gov account ( https://github.com/CMSgov/healthcare.gov ). (Edit: No, this isn't the marketplace code, but the fact that they share a Github account is some evidence the marketplace could be reusing/extending CMS.gov code).
That's only the static information and FAQ site, not the marketplace.
From the readme:
>This project does not include any source code for the Federal Health Insurance Marketplace (the online systems located under www.healthcare.gov/marketplace.