The DDG is the classification I'm using for Heavy Destroyers, which are a sub-class of Destroyer Leaders and Destroyers. Essentially some nations are taking the option of cutting out destroyers and destroyer leaders and going for a single class (mostly big nations that can afford it - think U.S., Chinese, and Russian construction versus some of the smaller nations building frigates). These Heavy Destroyers combine the numbers of Destroyers with capabilities of destroyer leaders. But seeing as they aren't leading regular destroyers, that name wouldn't make much since so they got the name Heavy Destroyers classified as DDG.
Ah, an extra set of eyes and ears for a little "Im in your backyard, stalking your business"! Btw, by the looks of the cover from that one revealed cannon, I assume that there are two more hiding on the dorsal hull. Also, by the looks of it, it seems to me that these guns cannot be extanded and as such have only a narrow firing arc, but correct me if I'm wrong.