GZIP and ZIP are different in that the archive format is different. The code supplied only supports ZIP archives.
However there is GZIP support already built in - look for the associated GZIP... stream classes in the API. These work well (I use these in applications).
One caveat, I have not specifically tested this, but I don't think the supplied code supports a GZIP archive, which you create when you have multiple files compressed sequentially using GZIP and packaged as a single GZIP file. In this situation, I think the standard stream processing will only provide you access to the first of the files.
