Might I suggest you actually read the contents of opkg-unpack before making such comments?The -z option is only required when creating packages which most users wont be doing.
Possibly it would be, but tar appears to hardcode /mod/bin/gzip rather than just gzip and therefore the version in /mod/bin/busybox/ is not found.For normal package install the version of gzip provided by busybox is sufficient so a dependency is not required.