Google recently stop using the Less Secure Apps for access to the smtp relays on May 30th 2022 Less secure apps & your Google Account. You will need to enable MFA and then create a password for the 3rd Party App to be able to use smtp.gmail.con as the email relay.
You can use this link to enable 2FA
This is the other link you will need as well to have the 3rd Party Apps be able to log in with.
Use the account and the app password you just created to resave the mail configuration in the appliance's UI. With that done, alerts should start being sent appropriately again.
Adding Note from Google. This currently only applies to the free version of Gmail at this time.
To help keep your account secure, from May 30, 2022, Google no longer supports the use of third-party apps or devices which ask you to sign in to your Google Account using only your username and password.
Important: This deadline does not apply to Google Workspace or Google Cloud Identity customers. The enforcement date for these customers will be announced on the Workspace blog at a later date.