It can run as a VPN, to funel and block unwanted WiFi or data usage from apps. Lots of settings and options per app to block WiFi and/or data. You can also set it so an app will allowed only if the screen is on (good for weather, lemmy or email updates)
As for gmail, it sounds like that would be setting within gmail, however I just briefly looked and did not see it. Personally I switched to K-9 email, it works great for me.
I wonder if gmail follows the system data saver or something stupid (If you happen to have that on)?