As crazy and esoteric it may seem, syncing your Apple AddressBook to Google really only works when your Mac is synchronized with an iPod or an iPhone. Just plugging such a device and syncing with iTunes triggers the sync to Google. You can also edit a file on your Mac instead to get the same result, as describe on Lifehacker.com:
Change the "Family ID." value to 10001 in ~/Library/Preferences/com.apple.iPod.plist

Thank you Gina Trappani and Josua Ferris!