drupal_add_css does now work when in cache mode
While you are using the version 7 drupal_add_js, the snippet you posted appears to be using the Drupal 6 arguments for drupal_add_css. You might also need to set the every_page option to true so that it is aggregated. Maybe try:

drupal_add_css('sites/all/libraries/gips/css/gips.css', array('type' =&gt; 'file', 'media' =&gt; 'all', 'preprocess' =&gt; true, 'every_page' =&gt; true));


You could also have a look at the group and weight settings on the api page linked above to see if they will help.