Cannot add gmail to unmonitored apps (battery usage) to get notifications in battery saving mode
I don't know if this will work since I have a LG G6 and am running Oreo.  So, the way I found to allow Gmail to battery saver mode was this:
Settings --> General --> Battery --> Power Saving exclusions
Scroll down to Gmail, then click on the switch to the right.

Tested by sending myself an email from another account and I got the notification!

Source:  Had to do it myself.