You can download this extension here.
The extension hasn't been heavily tested. If you are experiencing any problems, please open an issue.
If you already had a Gmail app, you might have to remove it and add it again. You can do so from the chrome://apps page.
You might have to manually enable notifications for the app. You can do so from the chrome://apps page. Right click on the app you previously created and select "App info". Under "Privacy and security" settings, make sure to allow notifications.
This extension does not work with the Gmail app downloaded from the web store. Make sure to follow the steps outlined in Setup to install your app. Do not install the app displayed below.