As far as I'm aware, the CRC check is part of the GZIP wrapper, not part of the actual compressed data in DEFLATE format.
So you should be able to take literally just the bytes that are the compressed data stream, ignoring the GZIP header and CRC at the end, and pass it through an Inflater.
In other words, you need to take just the bytes corresponding to those referred to as "compressed blocks" in the GZIP File format specification and try to decompress using a Java Inflater object. A little bit of work but possibly less than re-compiling the GZIP code as Greg suggests (though his option would also work in principle).