The compiler and runtime are an integral part of GCC, the GNU C compiler. The GNU C compiler can also compile C++ and C code.
In 1992, Dennis Glatting wrote the first gnu-objc runtime, and Richard Stallman wrote the second.
The version that has been in use since 1993 is the one developed by Kresten Krab Thorup; when he was a undergraduate student in Denmark. Kresten then worked at NeXT for a few years, and now he's getting his Ph.D. in Denmark.
If you have the sources to the GNU C Compiler (GCC), then you have the GNU Objective-C compiler and the runtime. The current version of GCC is 2.7.2, available at prep.ai.mit.edu and at mirror sites.How To Find GNU Software.