Using a Gmail account to send alerts from remote devices usually works fine. On occasion Google – for whatever reason – decide that this connection (that has been working fine for months) is now a suspicious login and blocks it.
Most of the time simply logging into the Google account and acknowledging (in Security settings) that the activity was genuine allows the mail to start flowing again. However you sometimes have to go an extra step and unlock the account. Retry the send of messages and you should be all set.