I do have plenty of production experience with GAE and is very different from yours. But our experience is from 2014. Since that period we were only affected by a single outage, on the 6th of Dec 2015 related to Google authentication against it's apis for around 17h.

Google seems to be giving a lot of love to GAE recently, especially with Managed VMs. They have delivered substancial improvements to the reliability of the platform.

EDIT: note that the issue I mentioned only affected 1.29% of the apps on GAE according to Google.