Since some others asked the same questions already. I think one should shed light on Google AMP. Google AMP uses special marked up HTML and restricts the site builder to only a few web elements. Thus, in principle writing an AMP theme is enough. However, since AMP uses different markup for image, video and embed tags (see https://www.ampproject.org/docs/get_started/about-amp.html ), you need at least a plugin (or a shortcode extension), which “converts” the standard HTML form of images, videos etc. into the AMP format. Roughly, speaking this can be done, without touching the core, but you can open an issue https://github.com/getgrav/grav/issues for your request.
There is only one thing, which remains: Why using Google AMP? Well, the main reason is that you already have a big site bloated of with a lot of JS (page load with >1MB) and you now want a fast website back. If you carefully craft your website and only use those libraries/functions/features you really use, then your website is automatically fast, without sticking to any third-party library or a hosting service . After all your pages are cached on a Google server for AMP, but you can do the same with a CDN of your choice.
All in all, I see no reason to use AMP for the moment. Clearly, it has advantages, but the price you pay is restriction of yourself. Better design a website/theme which is fast (and accessible) for everyone and not only for a small minority.
Grav can help you out here. It is fast by design and with the right plugins, you have the flexibility of a CMS with the speed of static webpages 