Please star this:
https://issuetracker.google.com/issues/35900034 (it's not created by me, I just came across while reading trough the archives).
As you may or may not be aware SSL support on Appengine is a mess with cryptic error messages, thing appearing and disappearing (seemingly) randomly, undocumented limitations (like the lack of SAN support), etc. Given that there are at least two "free as in money" options for Appengine to support transparent SSL for any domain (Let's Encrypt and I think Google is also its own CA - so it could issue any certificate they like), it would be great for SSL for verified domains to be just a checkbox in the cloud console.
Please star!