Rabbeinu Gershom, whose takkanot are considered binding by many Ashkenazim, is often the earliest name which I hear being cited in relation to the Ashkenazi mesorah. From whom was his mesorah, if known? Can it be traced back to Chazal?
From this article on early roots of Jewish life in Germany, it sounds like Rabeinu Gershom himself wrote about Rabbi Yehudah ben Rabbi Meir HaKohein (aka Rabbi Leontin) in one of his teshuvos, that it was "...Rabbi Leon who taught me the majority of his learning."
The Shaim HaGedolim from the Chida brings three opinions regarding who his teacher was. One is Rav Hai Gaon, another is Rabbeinu Chananal, and the third is Rabbi Yehuda Ben Meir HaKohen HaZakein who is called Rabbeinu Leontin.
The entry for the latter has no biographical information, but under Rabbeinu Leontai it says in the notes that this is an Italian version of the name Yehuda, and he is referenced that way in Hagaos Maymonis.
Maharshal writes that based on his references, Rabbeinu Gershom learned from Rav Hai Gaon, who was the son of Rav Sherira Gaon (who in turn has a whole letter in which he lists the complete chain of all the teachers until Moshe Rabeinu):
Sherira Gaon transmitted the law to his son Hai Gaon (born 956), who was last of the geonim. Rabbenu Gershon Meor ha-Golah (died 1040), Rabbenu Hananeel, and Elijah ha-Zaken, who was brother-in-law of Hai Gaon and brother of Jekuthiel, received the traditional law from Hai Gaon and from Jekuthiel.
A very interesting teshuvah by the way. Worthwhile to read, it is translated to English on Sefaria!