Lewis & Short

grĕgo, āvi, ātum, 1, v. a. [grex], to collect into a flock or herd, to gather into a host, to collect, assemble (poet. and only postAug.): Levitas ad se gregavit, Sulp. Sev. Chron. 1, 19, 6: Idaliae volucres caeloque domoque gregatae, Stat. Achill. 1, 373: oves, Paul. Nol. Carm. 17, 200: solus qui caedibus hausi Quinquaginta animas: totidem totidemque gregati Ferte manus, all together, Stat. Th. 8, 668 (al. gregatim).