During his lifetime R' Gershin was a recognized authority in Talmudic law. R' Gershon authored the sefer Avodas HaGersuhni.
The ritual inquiries directed to him while rabbi of Metz from western Germany and Alsace-Lorraine show that after his advent in that city he was really the spiritual and intellectual authority for the Jews of those countries. It was mainly in Metz that he exercised influence as a teacher. Ashkenazi was revered and loved by a large number of pupils whom he had the power to attract to himself. Chief among these was Rabbi David Oppenheim.