You probably mean Rav Saadya Gaon.
Rav Crescas actually says that although it doesn’t make sense to him, since they say that according to Kabbala it exists he’ll accept it.
Sam, I purposely chose free will, since that is an Ikkar according to the Rambam but not all that important according to everyone else. The Chovos Halevavos quotes those who don’t hold of it, and doesn’t toss it out the window. The Maharal says regarding the Maase Hashem’s Shita of Siluk Yedia, that it is better to hold there is no Bechira. The Rambam himself doesn’t make it into an Ikkar, only S’char Ve’onesh. Although, the Gemara Bava Basra does say that saying Barasa Tzadikim Barasa Reshaim is not correct.
I didn’t call Gilgul an Ikkar in Judaism, but tossing out the words of plenty Rishonim and most Achronim and the Zohar Hakadosh as a Goyish idea, does bump with an Ikkar.