Download the latest version of the GDiscord module from the releases page. The module you need depends on what OS and version you're running (eg. Windows on 32-bit should download gmcl_gdiscord_win32.dll). If you're running the 64-bit beta, make sure you download the 64.dll!
Copy this into lua/bin/. That's all!
With this module installed, your Discord status should automatically update when playing on supported servers.