Q:
How do I unpack downloaded files with .gzip file extensions.
A:
You can't simply rename the file with a package extension. The file is a gnutar archive which has been compressed with gzip. In order to install it you must first type at a unix shell prompt (in a Terminal window):
gzcat OmniWeb.app.tar.gz | gnutar xvf -
This will extract the application from the gnutar archive.