SOLUTION
download
components\com_virtuemart\sublayouts\related.php
change the ,'target'=>'_blank' to 'target'=>'_self'
echo JHtml::link (JRoute::_ ('index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=' . $related->virtuemart_product_id . '&virtuemart_category_id=' . $related->virtuemart_category_id), $thumb . $related->product_name, array('title' => $related->product_name,'target'=>'_self'));
Put the changed file in templates/gk_storefront/html/com_virtuemart/sublayouts so that you can still update virtuemart