I'm shelving this as "Arthurian" even though it doesn't actually mention Arthur, or much to do with his mythos for that matter. It does very briefly mention the Battle of Badon, however - the first source to do so, I believe - and as a result it seems to be frequently listed as the earliest piece of Arthurian literature. That's certainly why I read it.

Everything else is ranting. Seriously. Gildas was apparently one of those monks who approved of pretty much nothing, and most of "The Ruin of Britain", included here, is precisely what you'd expect from such a title. Things were bad and getting worse, and it was all everyone's fault, moral decay of society, falling away from religion, blah de blah blah. The rest of the sources are pretty much religious in nature - Gildas ranting, still, against corrupt and ineffective priests, or providing instructions on how long they're supposed to starve themselves for various offenses from accidentally vomiting up a communion wafer to bestiality (I shudder to think what went on in his abbey). There's a lengthy panegyric in there about martyrs, of whom he actually does approve, with a strong undercurrent of "You should be so grateful for the chance to be eaten by lions, you hell-bound disappointment", but for the most part it's mainly fist-shaking, and is admittedly quite amusing. If he'd lived today he'd be writing indignant pieces of outraged offense for whichever tabloid would put up with his sneering at their after-work happy hour.