These are the reasons for gmail Notifications Not showing:

1. The app notification settings have been turned off
2. Your Android device's system notifications have been disabled
3. Following the update, there was a system bug
4. Incorrect Label synchronization settings
5. The energy-saving mode is on
6. Gmail's app hasn't been updated in quite some time
7. You don't use Gmail as your main email account
8. The Gmail/email sync feature is turned off
9. Significant amount of data and cache in the app