The point is, for this to work, your system has to be connected to the internet. And if that's the case, there's no material difference between hosting it on prem and hosting it on GCP (where Google can promise you that it won't exfiltrate data). That's if you trust Google. And if you don't (and you shouldn't - it's an ad / mass surveillance company first and foremost), your only option for sensitive data is to self host and air gap.