If I select the option to sync my Address Book to Google:
and I delete a contact on the Mac Address Book (or from my iPhone), is the contact automatically deleted in Gmail?
And what about vice versa?
Yes, that is what sync is all about :)
You can try export your Mac Address Book as a backup, and then try the setup and see it for your self.
You can however also do it the other way, by exporting your Google contacts if this is your main contacts, and then try a sync setup.