For about the last 6 months, my gmail will not play notification sounds unless I open it. This seems to be a bit pointless as I can see new emails because I've opened the app! Not a perfect fix, but I've sort of got around this by installing another email app. It's FULL of ads but I never use it, just use it for the notification sounds. Anyone else having this problem?
TBH Why are you still using Gmail. There are way better email apps out there. Including the Samsung stock email client. Also, if you check on Google Play, there are many negative reviews on there. Just because it came with the phone, doesn't mean it is the best option.