Everything points to Apple deprecating 32-bit (architecture i386) code at some point. This is because they took advantage of switch from 32-bit to 64-bit code to add features to the Objective-C runtime, making 64-bit runtime more featureful.
I have no confirmation for this, but judging from how PowerPC transition was handled, and how 64-bit code deprecates loads of ancient technologies, this is probably not far off.
Although I do have 32-bit GameKit on my machine, it would not surprise me at all if a future release of OS X might contain no 32-bit code. And since all Macs that contain GameKit must be able to run 64-bit code (a requirement for Mountain Lion), the best advice I can give is to add a 64-bit version of the app. I have no idea if MonoMac supports 64-bit code, but that's the direction you should go to.
I suspect an OS update (such as the upcoming 10.8.3) might install a version of GameKit that happens to contain 32-bit code. Or it might remove 32-bit code for the rest of us. But I would absolutely not count on Apple maintaining or adding new features to 32-bit code, precisely because many features that they like (such as ARC or synthesizing ivars) can't be supported by the so-called 32-bit Objective-C runtime without breaking binary compatibility.