When you use two-factor authentication to sign in to your Gmail account (or to "Sign in with Google") you access your account with your email, password, and a verification code generated by Google Authenticator or other authenticator apps (such as Duo).
You might get an error like the one that follows when trying to sign in to Gmail with your Google password.
Authentication failed. Please check your username/password and Less Secure Apps access for mail@example.com.
Server returned error: "534-5.7.9 Application-specific password required. Learn more at 534 5.7.9 https://support.google.com/mail/?p=InvalidSecondFactor l25sm248619lfe.188 - gsmtp , code: 534"
When the service you're trying to use your Gmail account with doesn't allow you to "Sign in with Google," you need to create an app-specific password as detailed in the support Url provided by the error message.
This app password
You'll get an app-specific password like this one — dbkdwckcplvgaktc — that will let you log in to the authorized service with your email and this password.
In my case, I use this password to be able to "Send as" from Gmail from an email address that has two-factor authentication turned on.