How to synchronize gmail calendars in davdroid 1.10.1.-gplay using caldav in android 6.0.1?
The current Google API /  OAuth method is not supported by DAVdroid but it's still possible by using the deprecated format:

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.