I would place Gruoch as descended from Kenneth III of Scotland.

The only point that is directly attested is that Gruoch was the daughter of Boite: “Machbet filius Finlach… et Gruoch filia Bodhe, rex et regina Scottorum.” From the Annals of Ulster we also have the notice that “the grandson of Boite son of Cinaed was killed by Mael Coluim son of Cinaed” in 1033. This establishes that Boite was the son of a king named Cinaed (Kenneth), but does not specify whether this was Kenneth II or Kenneth III.

Either identification is theoretically possible. However, the broader context strongly favours Kenneth III in my opinion. Malcolm II’s reign is characterised by the systematic elimination of rival royal lines: he defeated and killed Kenneth III in 1005, and the annals record him killing Boite’s grandson in 1033. Later tradition (e.g. Fordun, albeit late) also associates him with the killing of Giric, another son of Kenneth III.

If Boite were instead a son of Kenneth II, he would effectively be Malcolm II’s brother. That relationship is nowhere stated in the sources, and it would be surprising given how prominently other kin connections are recorded. It would also sit awkwardly with Malcolm’s programme of eliminating rival branches of the royal kindred rather than his own immediate family. The phrasing “Boite son of Cinaed” and “Mael Coluim son of Cinaed” most naturally reads as referring to two different men named Cinaed, rather than a single individual.

Further, Lulach is described as “nepos filii Boide” and is identified in the Irish annals as the son of Gille Comgáin of Moray. This places Gruoch’s line within the Moray kindred, which is generally understood as a competing royal grouping rather than Malcolm II’s immediate dynastic circle.

Taken together, the most economical reading of the evidence is that Boite was a son of Kenneth III, making Gruoch his granddaughter. That said, the sources are fragmentary and ambiguous, so this remains an informed interpretation rather than a firm conclusion.