In order to enable the email notifications on your MetaTrader for your Gmail account, please follow the two steps explained below:
1 - 2-Step Verification must be switched off;
To do so you must log in to your Gmail account, go to Sign-In & Security -> Signing in to Google -> Password and sign-in method -> 2-Step Verification -> and disable it.
2 - Allow less secure apps;
This can be found in Sign-In & Security -> Connected apps & sites -> Allow less secure apps -> enable.