Occasionally on this list, someone asks about open source email &
calendar that can be used with LibreOffice. Here are instructions for
using Google Calendar with Thunderbird. You can use the Thunderbird
Add-Ons manager to install both Lightning and "Provider for Google
Calendar", instead of going to a web site and downloading the package.
This also works with the email program in Seamonkey, but Thunderbird
works better with LO.
https://support.mozillamessaging.com/en-US/kb/using-lightning-google-calendar