Cloud.gov is a PaaS based on Cloud Foundry. The idea is that it can run on pretty much any IaaS provider (as long as the Cloud Foundry plugins are written), certainly not limited to AWS.
We are using AWS GovCloud for our base IaaS layer, but we run a PaaS layer on top based on Cloud Foundry... That's the actual cloud.gov service being provided, the location on AWS GovCloud is just an implementation detail.