G-zip can be activated in the Joomla Global Configuration, on the Server-tab. Look for "Gzip Page Compression" and set it to Yes:
If you switch on Gzip in Joomla, all pages are zipped on the server-side before they are sent to the browser. Your browser then unzips the content again and displays the site. Gzip is supported pretty well nowadays, even Internet Explorer version 6 and up, so there is nothing to hold you back from using it. And it can give quite some gain in webspeed. You may gain 50% or more in bytes to be downloaded. Just try it out and check the result with one of the on-line checks for webspeed.
Also you can check on-line whether the Gzip process works nicely. Check your site on www.gidnetwork.com/tools/gzip-test.php or www.feedthebot.com/tools/gzip/. You can also see what the compression rate is then.