There aren't really any good libraries for it. You can compile straight ObjC on linux fairly easily with gcc or clang (use -lobjc), but the thing is without NSS

There aren't really any good libraries for it. You can compile straight ObjC on linux fairly easily with gcc or clang (use -lobjc), but the thing is without NSString and all of the foundation you'll end up doing a lot of work. And yes, you can get into GNUStep, but that's a whole nother ballgame.