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Forcing NeXT to release the Objective-C front end for GCC as Free Software was something of a PR coup for the Free Software Foundation. Unfortunately, it was not so good from the perspective of long-term cross-platform support. The code that NeXT released was terrible and, for the last ten years, has been largely ignored by the GCC team.
GCC has moderately good support for NeXT-era Objective-C, and also supports the newer exception-handling keywords. The GNUstep project provides implementations of the Foundation and AppKit frameworks that are the core of NeXT and Mac development.