Besides the Responsa, but closely related to them, we have the lesser Halakhoth of Yehudai Gaon of Sura (8th century) and the great Halakhoth of Simeon Qayyara of Sura (not Gaon) in the 9th century.
He was also a Talmudist of high repute, and wrote glosses on various Tractates, Responsa and other legal works.
He was author of many "Responsa," but his chief work was liturgical.
He wrote glosses to the Talmud (tosaphot) and many Responsa of the utmost value for historical research.
His brother Francesco (1418-1483) was also a distinguished jurist, and was the author of Consilia seu responsa (Pisa, 1481); Commentaria super lib.
Individual Geonim produced valuable works (of which later), but what is perhaps most important from the point of view of the development of Judaism is the literature of their Responsa or answers to questions, chiefly on halakhic matters, addressed to them from various countries.
Mordecai was also the author of Responsa.
The latter was a prolific writer of great influence, chiefly known for his Responsa, but also for his halakhic treatiseE, hiddushin and tosaphot.h.