I have looked at it recently and it seems Iran is blocking GCP, not the other way around. Not sure if Google keep a doc up to date with who blocks them.

From my past experience, I can say that Google Cloud services (e.g load balancers) by default blocked traffic from ITAR sanctioned countries. Not just blocking people in those countries from becoming customers of GCP, but blocking them from accessing content hosted on GCP.

I didn't know how that situation had evolved since I last used GCP.

Why do you think it's Iran doing blocking?

That's not what Google says: https://support.google.com/a/answer/2891389?hl=en