Missing apps in settings > notifications

You're welcome.

I just tried installing the Gmail app on my iPhone. After I have set up an account, I am asked if I want to allow the app to show notifications, after I tap Allow - then the app can be seen in the list in Settings → Notifications.

Have you set up an account in your Gmail app, if not then do this first. Or try deleting the app, then restart your iPhone and download the Gmail app again.

If the apps you are missing in Settings → Notifications have not been opened before, they usually do not appear in the list. When you open a new app for the first time, the app asks if it can send you notifications. There you can answer yes or no. No matter what you answer, the app will appear in the notification settings afterwards.

You're welcome.