It is plain to see in the Zohar Hakadosh. Even non-Mekubalim accepted it, albeit begrudgingly, after the Zohar Hakadosh.
The Gemara is not unanimous about Moshe Rabbeinu not getting an answer. The Ramban says that this is not something that you can figure out on your own. It is only know Bikabala. Moreover, while Gilgul answers the question from one angle it doesn’t answer all the problems.