I have experienced an issue with the GC app where the app crashes when selecting "Allow" on the popup when navigating to Settings -> Smart Notifications and subsequently not been able to receive notifications on my device (Forerunner 35). Below is a workaround which worked for me. I hope this is helpful to someone else as well.
Uninstall the GC app and then reinstall and sign in, but DO NOT ADD your device during the initial setup/configuration steps of the app. Skip that step. Once the app has been installed and you have signed in, go to Settings -> Smart Notifications. After about 2 seconds, you are prompted to allow access to notifications (but this is through the Android OS and not through the app.) Once you have granted access, you can set the app permissions in the GC app. Now add the device in the app (Garmin Devices -> Add Device). Lastly, make sure that your Garmin device also allows notifications.