Here are the most common ways to provide that second "key":
Think of g.co/verifyaccount as Google's digital bouncer. It is annoying when it stops you, but it only exists to keep criminals out of your data. By understanding how it works, you turn a stressful moment into a quick, 60-second security check.
Checked that both a phone number and a non-Google recovery email were active [6].
The screen went white for a moment. The loading circle appeared again. Then, the familiar, comforting interface of his Gmail inbox loaded up.