Hello,
When I enable the template built-in CSS compression feature, it breaks the main menu layout : as shown in attachment only the first menu item of my Main Menu is shown. (Same behaviour with external optimization plugins when compressing/merging javascript or css).
My site is Joomla 2.5, last version of template and modules. I tried it out with disabling any other menu than the GK one and it is the same behaviour. Both GK and core system cache are disabled for now.