That position may have been the same one that vortigern occupied, a commander in chief selected by some council that Gildas alludes to.
Yeah, I've seen it suggested that Vortigern was actually just a local king and his "counsellors" were just an ad-hoc gathering of rulers trying to sort out the Saxon situation, but Gildas seems to assign him a more important position, judging by the amount of vitriol he reserves for him.
Another thing I've just thought of -- Gildas calls the Britons citizens (
cives) and the wars between the British kingdoms civil wars. Now maybe he only means this in the sense that the Britons are united by a common language and culture, but I don't think there are many parallels of people using the term civil war to refer to conflicts between culturally similar but politically distinct entities. (E.g., I don't think the ancient Greeks or mediaeval Italians called wars between their city-states civil wars, even though they clearly had a sense of cultural distinctness from their neighbours.) So I think Gildas probably means that there
is a central government with authority over the local kings, although given the impunity with which these kings wage their civil wars it seems that its authority was pretty nominal by the time Gildas was writing.