From context, it may have a lot to do with tra'ka' - "And a Ginnungagap for those that say different!"
The Ginnungagap is viewed as a sort of Dwarfish Hell, or would be, if they believed in one; quite possibly, for a race predicated on order and its maintenance, being thrown into a place of utter chaos would be a sort of Hell. Ardent, the rogue grag with a grudge and an urge to vengeance, has this fate wished on him several times: most notably by Albrecht Albrechtsson when he usurps the Low Kingship in a coup d'état
According to the History Monks, the Ginnugagap is the gap between the end of the Discworld and the beginning of the next creation. Lu-Tze believes that the dawn of an entirely new world would be a good thing, if only there were some way of crossing the Ginnugagap before it happened.
Wikipedia defines Ginnungagap as the primordial empty abyssal void in Norse mythology, the primordial Chaos from which all things began and into which all things will eventually fall. It is possible Sulien was wishing the Grags to Hell as well as describing how little he cares for their views.
It is also worth noting, from a cultural and linguistic perspective, that in the Nordic countries of the Roundworld, Norway, Sweden and Denmark, the very worst possible **** Off! is "Go to Hell!". This is still fighting talk in Denmark and the intensity of this epithet is often not completely understood in nations that draw their ultimate oaths out of scatology rather than religion. Indeed, the English subtitles of a show like Borgen fail to realise this, and English viewers watching tend to wonder why the Danish cast are nearly coming to blows over what sounds like a very mild epithet (by English standards), where frequent expletives translated as "F*** Off!" are used in much less fraught discourse or heated situations. (These exist in the Nordic languages but are mild profanities, along the lines of "Sod Off!" or "Bugger Off!" in English.).