When you create an Alert Notification Profile in the gateway, you can then add Distribution Groups, which asks for a name, and an expression. That expression uses the same expression language that Ignition uses, but also has special alert properties you can reference. The following link, under the “Notification Settings” header, has a list of the special alert properties you can reference:
inductiveautomation.com/support/ … b=severity
That expression will act as a filter for that Distribution Group, and only if the expression evaluates to true (or “valid”) will the members of that group receive an e-mail.