Maybe something needs to be changed in your SEF component.
I am facing a different problem allthough.
When I am clicking on a product in any other module everything is OK.
When I click on a product in the GK4 module the product appears but the modules on the left or on the right do not appear.
I noticed that when clicking on the product in "Latest Products" module the URL is:
http://www.mysite.gr/v2/index.php?page= ... tegory_id=75&flypage=flypage.tpl&product_id=1106&vmcchk=1&option=com_virtuemart&Itemid=2
but when I click on the GK4 module the products URL is:
http://www.dimitriadis.gr/v2/component/ ... tegory_id=null&flypage=flypage.tpl&product_id=1106
The problem is that the module does not get the category ID (as you can see in the red script above)