I think in Apps > App permissions or somewhere you can amend the level of battery saver for each app.
Yea you can, very similar to EMUI: App Battery Saver and then set to 'No Restrictions'. Which I did for Gmail and Google apps. I read a few people also suggested setting Gmail under Autostart within security but Gmail isn't listed under normal or system apps (and I've seen a few people use that .js app to remove the Gmail app in the image and reinstall from the Play store to get around that).
But for whatever reason even changing those, Gmail has not been pushing notifications until some point today. That was was after a few hours playing with the options.