After a few minutes of lurking around my phone changing various settings, I finally stumbled upon a fix to this, but I'm still not sure which part of my experiments actually restored this function.
The solutionNow we are gonna go through some checks about which options are enabled and which ones are disabled on your phone, so I'm just gonna list you the steps by order that I tried starting from the first to the last one.
- Open up your Gmail app, head into Settings and then go to your Gmail account. From there, make sure the setting "Sync Gmail" is enabled aswell as the "Notifications" one.
- Go back, and now head into "General settings" and from there, tap on Message actions, then make sure that the first option "Always show" is enabled.
- Go to Settings -> Accounts -> Google -> Tap on your e-mail and from there make sure that the option "Sync Gmail" is enabled.
- Go to Settings -> Application manager -> Swipe to the right side with your finger twice and you should be in the "ALL" category. Scroll down to the Gmail app and tap on it. Now in there you will find the option "Show notifications" enabled, now disable that option, then enable it again.
Now just wait to receive an e-mail and you should see the notification appear, or you can simply send an e-mail to yourself to test.
I hope this helped you restore your notifications incase they were gone. If it helped, please leave a comment as feedback.