If you do not own an iPhone, you may not see the option in Address Book to sync contacts with Google's Gmail. If you are
not seeing the option in the preferences of Address Book > General, then read the 3 steps in the follow post on how to enable the Gmail contact option box:
http://lifehacker.com/393855/enable-google-contact-sync-without-an-iphone-or-ipo d-touch
(Excerpt:)
Here's what you do:
1. If you don't have the Property List Editor installed, download the shareware
PlistEdit Pro and install.
2. Open up ~/Library/Preferences/com.apple.iPod.plist and save a backup copy of it. Then, expand the tree to reveal "Family ID." Change that value to 10001, as shown above.
3. Save your changes. Launch Address Book and open the Preference pane. The Google contact sync option will be staring right at you.
Next, open iSync, choose iSync > Preferences, and then select “Show status in menu bar.” Quit iSync. Click the Sync menu in the menu bar and choose Sync Now. (taken from Address Book Help > Synchronizing with Google Contacts).
Voilé!