Are you referring to the app called "Email" with an envelope icon and red border? If so, I used to never use this app, but instead preferred the Gmail app. But only because I recently got the Samsung Galaxy smartwatch did I start using the built-in Email app which pairs well with my Galaxy watch for notifications via Bluetooth from phone to watch. If I recall correctly, for whatever reason, the Gmail app doesn't seem to send email notifications to my Galaxy watch, but the built-in Email app does. However, that said, I do get email notifications from the "Promotions" folder (eg: Bed Bath & Beyond store coupon offers, etc) from my phone to my Galaxy watch. I don't know if there is any way to filter out "Promotions" (or Spam) emails sent to my watch. As for the built-in Email app, I have both my Gmail and AOL emails set to show up on my Galaxy watch by enabling (checking off) the "All accounts" option.
EDIT: On second thought, the reason I now started using the built-in Email app is because the only email app that I can add to my Galaxy watch is referring to the built-in Email app. The Google Email app is not an app supported or offered in the Galaxy watch, hence I now use the built-in Email app so I can take advantage of notifications sent from my S9 phone to my Galaxy watch. I wish the Galaxy watch has the Google Email app...maybe in the future hopefully.