The GNUstep Objective-C runtime was designed as a drop-in replacement for
the GCC runtime. It supports three ABIs:

- The old GCC ABI, which provides support for Objective-C 1.0 features.
  This can be selected via the -fobjc-runtime=gcc flag in Clang or by
  compiling with GCC.

- The initial GNUstep non-fragile ABI, which was intended to be
  compatible with the GCC ABI, but provide support for modern Objective-C
  features. This can be selected with the -fobjc-runtime=gnustep-1.9 flag
  in Clang.

- The modern (v2) ABI, which provides richer reflection metadata, smaller
  binaries and reduced memory usage. This is selected with the
  -fobjc-runtime=gnustep-2.0 flag in Clang 7.0 or later.