I recently realized that gContactSync (among with other Thunderbird Add-ons) is unfortunately not compatible with Thunderbird 102. Looking for an alternative I came up with the following solution. To enable Google Contacts sync via nativ Thunderbird 102 function you just have to add a new CardDAV address book and use the following address and your Google Username. Please be aware to adopt part of the URL <username@gmail.com> with your own Google email address.

https://www.googleapis.com/carddav/v1/principals/username@gmail.com/lists/default/

Source: https://www.thunderbird-mail.de/forum/thread/90064-tb-102-adressbuch-mit-google-kontakten-synchronisieren/