The spec is here: https://github.com/ampproject/amphtml/issues/13457
Thanks! I see the link in the blog post is now fixed.
Re:Bringing AMP to Gmail
by SippinLean on Tuesday February 13, 2018 @10:40AM
(#16367925)
>To get developer preview access to AMP for email in Gmail, sign up on our site.
It's a subset of existing AMP components.
They say that the spec is available. But where? They link to https://github.com/ampproject , which is just a list of projects, none of which seem relevant.