Gildas was no prophet. In his time, the Saxons were subdued. It was about 570, which is also his likely death date, that they started moving again, and conquered most of what is today southern England. The lands north of the Trent did not fall until 615, and Devon later still. Gildas lived in, and described, a world where the Saxons were defeated; if he proposed a crusade against them (using an anachronistic term), itr was because it was the only way he could see to unite the British kings. Plus, there is some indication that the British had already started to use Saxon mercenaries in their wars, which was one of the things that would eventually lead to Saxon triumph, and of course a crusade would change that.