Hi,
If you take a look at the page source code, you will see that only one CSS file is included
index.php?fetchcombinedfile=css-6fe0726f2d107a47cecc2956917864b1
This is a "combined file" which regroup several CSS file in a compressed mode.
It does not have any link with the gzip compression of your server, it is long before that.
This is the feature that you should deactivated in order to see if the page is displaying better without.
And what I can tell you is that the "modal.css" file is include in the HikaShop product page (because you use the mootoos modal system).
This modal.css is wrong compressed by your template system and there is some strange code:
@-moz-document url-prefix(){.body-overlayed { overflow:visible}
Which break the rest of the CSS below, like the template.css file which contain all useful css rules.
So please, deactivate this feature to make the system work.
Otherwise you can use another popup system in HikaShop but if you joomla wants to display any kind of mootool popup, the page will break like this one.
Regards,