How can I get all of my Gmail contacts into my Mac Contacts?
On Lion/Mountain Lion, go to the Preferences on your Contact app, choose Accounts and then On My Mac, and there should be a checkbox Synchronize with Google, you will just need to provide your Gmail credentials.
Just as a precaution, make a backup of your contacts first. Just need to go to Gmail, select Contacts, then the More button, Export to download all your contacts to your hard drive.