Open Address Book, select an address book in the left pane, Tools/Export, name the (comma separated values, i.e. csv) file. Open gmail with your browser, click the small triangle next to 'Gmail' in the upper left, select Contacts, More, Import, CSV or vCard file, switch to Old Contacts, then select More/Import... select the csv file. If you would rather synchronize the contacts, use [https://addons.mozilla.org/en-us/thunderbird/addon/gcontactsync/ gContactSync].