Rated 3 out of 5 stars

As this appears to be abandoned, I can recommend a replacement by Emmanuel ROECKER (https://addons.thunderbird.net/en-US/thunderbird/user/emmanuelrkr/) in the "Open Google Calendar" (https://addons.thunderbird.net/en-GB/thunderbird/addon/open-google-calendar/) add-on. It's first rate and has a 5* review. GCT used to be worth 5*s but now it's incompatible with the latest versions, sadly I've given it a 3 "for old time's sake"