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CloseReview for Provider for Google Calendar by Sina Momken
Rated 4 out of 5 stars
Also TB has Google Calendar integration by default using CalDAV protocol but after searching through web I found out that this addon has some little more features and is easier to setup.
Plus this addon provided Google Calendar support before Google support CalDAV itself.
I gave it 4 star because it doesn't support Google Tasks syncing with Lightnings' tasks.
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