The Gesta is fundamentally a versification of Geoffrey of Monmouth's Historia Regum Britanniae in Latin epic hexameters.
It retains Geoffrey's overall sequence and structure, but expands upon those elements and stories which had the greatest dramatic potential, while treating other sections more cursorily.
William omits the Prophecies of Merlin section of the Historia, as Wace did in his earlier Roman de Brut.
William may have read Geoffrey's Vita Merlini, but otherwise does not intrude any elements of the (by then very copious) Arthurian legend into his adaptation of the Historia.
It is divided into ten books, each of which is prefaced by a terse summary of its contents, also in verse.