Minifying Javascript and CSS not only zips it but it adds other optimisations that are impossible by zipping.
For example, by minifying you can modify the name of a long variable. All the instances of that variable will then be only one character.
Another thing minification does it removing comments. This cannot be done by gzip.
Apart from that minification usually bundles various files into one thereby reducing the amount of requests
Apart from minification you should ALSO use gzip
