Copied from https://www.jeffgodwyll.com/posts/2016/letsencrypt
Let’s Encrypt is a free, automated, and open certificate authority (CA), run for the public’s benefit. Let’s Encrypt is a service provided by the Internet Security Research Group (ISRG)
This post won't go into details as to what Let's Encrypt is about. We should probably read the FAQs if we want to learn more.
Until the Google App Engine Team fully automate the process of using Let's Encrypt on Google App Engine or even provide some sort of API to handle certs, we'll probably have to find ways of either automating the process just a little bit or stick to some other easier cert authority.