Let's break it down a bit...
"Porter takes care of a lot of generic DevOps work for you (like setting up CI/CD, containerizing your applications, autoscaling, SSL certificates, setting up a reverse proxy)."
All of this is done for you on GCP with the aforementioned services.
"Porter Cloud for as long as it saves you time and development cost, but at any time you can press the “eject button” to migrate your app to your own AWS, Azure, or GCP account as you please."
Why add an additional service, and set yourself up for having to "eject", when you can just start off on the right foot to begin with?