Gmail two-step verification is an optional feature to your Gmail account that adds the more extra security to your Gmail account when you enable this service then any attempt to verify your phone number on Gmail must be accompanied by the six-digit passcode that you created using this feature.
To enable this service you must follow some steps to know what these steps are then visit at this informative post.
[Steps to enable Gmail 2 Step Verification Security]
You can also take the Gmail expert help to Enable this service on your Gmail account by calling at their toll-free number.
+44 - 800-051-3717 (UK)