I’m sure there’s been Mac games written in Objective-C, but there are certainly no cross-platform games written in it because vital bits of Objective-C aren’t a

I’m sure there’s been Mac games written in Objective-C, but there are certainly no cross-platform games written in it because vital bits of Objective-C aren’t actually part of Objective-C; instead, they’re part of the Foundation framework in OS X and iOS. You can compile Objective-C on whatever platform you like through GCC but you’re going to have to write your own memory management, allocation, etc among other things in order to do so. That said, you might be able to write a cross-platform game through open source Foundation/AppKit implementations like GNUStep and Cocotron, but keep in mind that they implement a version of Foundation that’s several versions behind now. The next closest thing is probably Swift alongside Apple’s FOSS reimplementation of Foundation for Swift, though I don’t know how well Swift works for development with OpenGL and such.