How can I reenable Gmail notifications after disabling them through Inbox?
You have removed Inbox app, but the issue remains. If Inbox did modified Gmail settings, then you have to tried the "usual" steps:


Clear Gmail's cache.
Clear Gmail's data (this step also clears the cache): It will reset the app to default. You will have to login again to your account as you did the first time you used the app.
Remove and re-install Gmail (if it was not installed by default like in some custom ROMs).
Remove all Gmail updates (it will allow Gmail to return to the state it had when you bought the device).