Benjamin Harley has written an applescript that merges Gmail and Address Book contacts.
It works by taking a CSV file of your exported Gmail contacts (which you need to create first), and comparing it to the contacts in your Address Book. Any contacts not found in the Address Book are added.
Contacts in Address Book but not in Gmail are added and the merged CSV file is then imported back into Gmail.
All of this and more is explained carefully in a ReadMe file 
for the script, which you should read before using the script.
As Benjamin explains, the script is a labour of love (he wrote it initially for his girlfriend) and is not widely tested. He welcomes feedback on how it works for you. You’ll find an email address on his web site.
ABGMerge is donation-ware and is available from Benjamin’s web site .
