Enabling Notifications - Send Email in the settings of an indicator is not enough to get emails on alerts. You need to set up Email share service also.
In Ninja menu click Tools - Options. In General click Share services to add Email share service.
Watch this video on how to add Gmail account: https://www.screencast.com/t/2VshigfRtcVF
Managing Share Services: https://ninjatrader.com/support/helpGuides/nt8/?general_section.htm