You might be getting GovCloud and AWS Secret Region confused. I've been told that I can access GovCloud if I cross the border from Canada, despite not being a US Citizen. (This came up in the context of providing FreeBSD AMIs.)
GovCloud customers only need be a US person or entity, beyond that any further regulatory alignment is up to the customer. AWS does not audit the IAM user base for nationality or any compliance requirements.
Disclaimer: I am an AWS Public Sector Solutions Architect.