If you used a login name and password to create your account, and want to add a Google Sign-in option, log in to your account and click on your name at the top right. Click on Add Google Sign-in beneath your email address and follow the directions. After completing this step you will have two different sign-in methods: (i) login name/password, and (ii) Google Sign-in.
You can remove the Google Sign-in authentication at a later time, by clicking on Remove Google Sign-in beneath your authentication information. After removing Google Sign-in, you can re-add it later, or add different Google Sign-in information as needed.