I find it unlikely that gzip and mv would be such memory hogs (even on HP-UX ;-). Perhaps the script is doing something resource unfriendly, like foreach (<FILE>), or perhaps the system()s are being kicked off with "&" to put them in the background.

Use top(1) to see how many processes are running and how much memory they are using. If indeed the command utilities are causing problems, you might find more recent versions on the HP-UX porting center.