Slitherine

The gold edition is the only panzer corps game that does not have a "Get Steam Key" next to it on this page: http://www.slitherine.com/members/games.asp?Filter=0.

Does that mean I cannot download it from steam if I buy through slith's store, or do I have to register all the individual keys separately?

You need to register separately the key in all DLC and expansions for Panzer Corps and then redeem a Steam key for each one.