I didn’t get notifications when I first used the app on Android 8.1, but after I close and reopen the app, the subsequent notifications do get sent. Please try this:
Android Settings > Apps & notifications > See all X apps > Gameflip > App notifications > On
Try toggling it off and on. You can also try reinstall the app, then open it, then go to the app switcher and close Gameflip. Reopen Gameflip and it should try to register you to the notification service again.