Portability of Objective-C

I am looking into learning Objective-C, but it looks very tied into OS X, i dont like that. Would anyone share some anecdotes of writing portable Objective-C code? Specificly, how does the GNUstep runtime compare with the one provided by apple?

>>9
msvc (assuming no winapi) compiles with gcc

gnu c code doesn't compile with msvc unless you really try to conform to their standards