Our only possibly contemporary source for Gwallog's life comes from two Middle Welsh poems honouring him attributed to Taliesin by modern scholarship.
[7] That Coel was truly the progenitor of these dynasties, however, is a matter of ongoing academic debate, since the only testimonies of this common descent are from texts written in Wales hundreds of years after the kingdoms they represent disappear from the historical record.
Next to nothing is known about Gwallog's father Llênog, who may have been the eponymous founder of a (possibly monastic) settlement called Llanllennog, the location of which is entirely unknown.
Gwallog occurs in a section dated to the reign of Theodric of Bernicia (d. c. 572 x 593), where he, together with Urien, Rhydderch Hen, and Morgan, are recorded as fighting against that Anglian king.
One poem states that 'Gwallog, horseman in battle, intended to make corpses in Erechwydd against the onslaught of Elffin [ab Urien]'.
[16] The other poem refers to a lost story about Gwallog losing one of his eyes to a goose, though it is apparently not meant to be a humorous tale, despite how it may appear to the modern reader.
[19] In Ystorya Gereint uab Erbin, a text of unclear relation to Érec et Énide, Gwallog is included as one of Arthur's 'best men'.
[20] Very late genealogies give Gwallog's sister as Dwywai ferch Llênog, mother of Deiniol, first bishop of Bangor; his daughter was supposed to have been Onnen Greg, married to one Meurig ab Idno, who is also mentioned in the saga poetry.