They are composed in the ancient Bardic measures, and were with one exception, namely an elegy on the death of his benefactor Lewis Morris, which was transmitted from the New World, written before he had attained the age of thirty-five.

All his pieces are excellent, but his masterwork is decidedly the Cywydd y Farn or "Day of Judgment." This poem which is generally considered by the Welsh as the brightest ornament of their ancient language, was composed at Donnington, a small hamlet in Shropshire on the north-west spur of the Wrekin, at which place, as has been already said, Gronwy toiled as schoolmaster and curate under Douglas the Scot, for a stipend of three-and-twenty pounds a year..