The basic process is to use an ssh client to login to a shell on your DreamHost server. Once you successfully connect, you will be presented with a command line from which you can execute various *nix command to operate the system, including manipulating files.
To do this you need to have your user authorized for shell access, which you can do from the Control Panel -> Users -> Manage users screen (just click the "edit" icon under the "Actions" column at the right end of the rows for your user). On the resulting screen, select the check box to authorize shell access for that user, and submit the form.
Once this is done, you can proceed to use the shell. Good instructions for finding your way around the shell can be found in the DH Wiki articles on the Shell.
Make sure you read the articles linked in that first articles, as they contain further instruction regarding the use of an ssh client, and other stuff you will need to get started.
Once you have managed to log in and execute a command or two (ls to get a directory listing, etc.), you will be ready to use one of the "zip" commands to manipulate your file.
you can get basic instructions for using "gunzip" from within the shell by typing:
man gunzip [enter] - hit the "enter key", don't type the "[enter]", and the "manual pages" for the gunzip program will be displayed.
From here, there is really no limit to what you can learn - the web is full of excellent unix/linux tutorials (several are linked withing the DreamHost wiki pages if you follow the links from the initial shell article).
Once you have looked around and reviewed that DH Wiki information, if you have a specific problem, I'm sure you will find additional help on these forums. Good Luck!