During the Bombardment of Genoa, he frescoed an "Allegory of Time" and sculpted a frieze in the drawing room of the Villa Gropallo. He later frescoed two vaults of the Palazzo Rosso to accompany two rooms completed by Piola. By 1689, he had frescoed the Brignole Sale drawing room with the "Myth of Phaeton" and painted a small cupola fresco in its chapel (both since destroyed). Gregorio and Piola worked in the same ornamental style, but differed in that, while Piola delineated each figure in his composition clearly, Gregorio took interest in "arranging twisting, elongating figures to soar through space in a swirl of arms, legs and draperies." (ibid., 8) He worked with Piola again in the Palazzo Granello, where he frescoed two rooms with "Cupid and Psyche" and "Neptune and Amphitrite".