You want to cross compile which means you need a cross-compiling toolchain. While Nim can cross-compile, it generates C/C++ which then needs to be further cross-compiled to the target arch.
Is the gcc you are using a 64-bit Linux compiler that runs on OSX? You might also want to check out [https://github.com/dockcross/dockcross](https://github.com/dockcross/dockcross) if a native OSX to Linux cross-compiler doesn't exist.
