This is a port of the decompression portion of bzip2 to the GNO environment on the Apple IIgs. See README.GNO for more information specific to the GNO port. These sources might be adaptable to run under other IIgs shells too, but I haven't done that.
Binary downloads are on the releases page.
Note that this is based on an old version of bzip2 with known security vulnerabilities, so don't use it on files from potentially malicious sources.