Webmaster level: Intermediate
Here’s how it’s done:
1. On the Webmaster Tools home page, click Manage site for the site you’d like to verify, then select Verify this site. If you haven’t added the site yet, you can click the Add a site button in the top right corner.
To do this, you must have "View, Edit, and Manage" account level permissions in Google Tag Manager.
2. On the Verification page, select Google Tag Manager as the verification method and follow the steps on your screen.
3. Click Verify.
And you’re done!
If you’ve got any questions about this verification method, drop by the Webmaster Help Forum.