Also not addressing your specific problem, but what makes you think multiple processes will speed up your task? Is the box multi-processor? Are all the files on different hard drives?

As far as I know, gzip operations are typically CPU bound, which means that their speed is entirely dependent on how fast your CPU can process. So adding another process will just make both take twice as long. This is also true if you are hard drive bound - adding more processes won't speed it up.