When we integrated GoodRelations, we skipped the category property for reasons I do not remember.

This should be fixed - simply add
<span>Domain: <a property="http://schema.org/domainIncludes" href="proxy.php?url=http://schema.org/Product">Product</a></span>

in the block for

<div typeof="rdf:Property" resource="http://schema.org/category">

This will also make this consistent with

http://www.heppnetz.de/ontologies/goodrelations/v1.html#category