Objective-C is a strict superset of C so porting it shouldn't be too hard provided gnugo doesn't hook into the OS too much. You'd just have to provide a few interface widgets to make it useful.
Be aware that you wouldn't be able to release it to the app store though, since it's GPL 3. The project's been open a while though, so I guess you could grab a version from when it was GPL 2 and port that (assuming you read "distributor" in section 3 as yourself rather than Apple; IANAL).