ObjectiveC2.framework implements the new Apple runtime APIs, introduced with OS X 10.5 on top of the GCC Objective-C runtime.  Its use is now discouraged.  This code has been merged into the GNUstep Objective-C runtime, which can act as a drop-in replacement for GCC libobjc.  You can find the code here:

svn://svn.gna.org/svn/gnustep/libs/libobjc2/trunk
