The current Google API / OAuth method is not supported by DAVdroid but it’s still possible by using the old format as mentioned by @daniu :
https://www.google.com/calendar/dav/[yourname]@gmail.com/events
username
password
Yet it requires the compromise Allow less secure apps: ON in the Google security settings.
However, delegates don’t sync.
Tested on OnePlus 3t, Android 7.1.1, DAVdroid 1.10.1.1.