it is important to understand that gzip and pkzip are completely different formats. well, they are both called zip
anyway, gzip is a stream zipper which just allows to compress a data stream. this is very useful for rim as they can compress their data streams with the BES or whatever.
if you want to store several files in one zip file you need a pkzip file, which means you need pkzip functionality and that is not part of the blackberry api yet.