Rated 5 out of 5 stars

This review is for the newest versions at the time:
Thunderbird 38.3.0
Lightning 4.0.3.1
Provider for google calendar 1.0.4
The setup was very straightforward. (you have to have the Provider addon installed and enabled)
File->New->Calendar->Network Calendar->Enter gmail address->Enter gmail password->Choose Calendars to sync

That was it, you are ready to go.
Avoid older instructions in thunderbird support using xml instructions and such, it is not needed.

This review is for a previous version of the add-on (1.0.4).