First install thunderbird (if not already installed)
sudo apt-get install thunderbird
Then add your existing gmail account
File->Existing Email Account
Now add support for syncing contacts
Tools->Addons
Search for 'google contacts'
Choose 'Google Contacts 0.7.12'
Now add support for calendar:
Search for Lightning, install it
Search for 'Provider for Google Calendar', install it
Follow this tutorial to add your calendar to Thunderbird
https://support.mozilla.org/en-US/kb/using-lightning-google-calendar
