I'm trying to implement this too. Seems like it is working on macOS but iOS does not. I've implemented the flow as described by @DTS Engineer GKGameActivity.checkPendingGameActivityExistence is false even if I open the a challenge to a leaderboard from the Games App. I have iOS 26.3, macOS 26.3 building with Xcode 26.2. Curiously the Debug > GameKit > Manage Game Progress window shows nothing on both iOS and macOS. The app shows up but nothing seems set up. The leaderboard is live but the activity is not in App Store Connect.