Contacts on your Mac are actually managed by Address Book, not Mail.
I'm not sure this will work, but it's worth a try.
1. Go to your Gmail Contacts and click "export" in the top right corner.
2. Choose the Outlook option for CSV export.
3. Use
this to convert the CSV file into vCard format.
4. In Address Book, choose File > Import, and get the vCard imported.
Hope that works somehow. At least it's a start at where you should be looking.
UPDATE: Sorry, I forgot the link. Now it's there.