Forums.grimreapergamers.com now supports push notifications on Android. Pending your mobile browser supports it. Currently supported ones are Chrome, Samsung Internet, Firefox, Edge and UC Browser.
To enable: Make sure that you turn on “Enable Notifications” under Live Notifications in your Profile > Preferences > Notifications. You must do this on your mobile browser.
You will then start to see notifications as people either Mention, PM or reply to a thread you are watching.
Apple Users: Sorry but this not yet supported as Apple has to be different and not use the standard Push API. Alternatively I would suggest downloading the Discourse App off the App Store. This will poll the site for updates and provide notifications that way.