If you want to host static sites at no cost on Google's global infrastructure, I suggest a look at "Firebase Hosting". It is a closer match for this need, works very well. It has some features that GAE doesn't, like the ability to auto-issue-and-provision an SSL cert for a custom domain. It has HTTP2 push.

https://firebase.google.com/docs/hosting/

Don't be distracted by the word Firebase - it is just a brand name in this context. You don't need to use the Firebase database, just because you use Firebase hosting.