how to gzip compress js/css in drupal 7
The easiest way is to go to Configuration > Development > Performance (admin/config/development/performance), and enable:


Cache pages for anonymous users
Cache blocks


And also:


Compress cached pages
Aggregate and compress CSS files
Aggregate JavaScript files


For websites with mostly anonymous (not logged-in) visitors, this is often sufficient. Note that if you enable compression in Drupal, you should not enable page compression on the web server level (Apache) as well.