Currently I only do job related work in C++, JVM and .NET.
Never looked too much into GNUStep, because it is not clear to me how far Cocoa and the latest language changes from Apple are supported by gcc and clang + GNUStep.
On my line of work it is the customers that decide the technology, not me, so they have a hard time thrusting Objective-C outside Apple's environment.
http://wiki.gnustep.org/index.php/Platform_compatibility
I also had a large-ish system running on AIX.