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.

There is an easier way to do this. Pull up your contacts in Gmail, select the "more" botton, click the export contacts option, it will give you the option to export them as vCards. Export as vCards to the desktop. Pull up mac contacts, click import and select the file you dropped on the desktop. You are done.