This feature requires you to be using the Discord desktop app for Windows or macOS, while you're playing GeForce NOW. It is not currently compatible with the web browser version of Discord.
Your user status will update within 15 seconds of launching or closing a game or platform. It can only show one item at a time and will show only what you launched directly from GeForce NOW.
For instance if you launch Blizzard Battle.net, then launch a game within Blizzard Battle.net, your user status details will only show the first activity as "Playing Blizzard Battle.net". However if you launch a game directly from GFN App > Library, it will display the game name in your user status.
If you're experiencing different issues, please let us know on the forums so we can check it out.