There is a Machlokes תנאים if גיד הנשה is מותר or אסור בהנאה
(benefit) Reb Yehuda holds that it is מותר and R. Shimon* holds it is אסור בהנאה.
The Bais Yosef (Y.D. 65) writes:
"that is best to refrain from having הנאה from the גיד הנשה because The Zohar פרשת וישלח writes that גיד הנשה is אסור בהנאה."
The Rema in Darkei Moshe writes:
he heard "(שמעתי) that the author of the Zohar was R. Shimon (Bar Yochai) the same one who is mentioned in the Talmud.
The Rema asks, If this is true, then why should we care that the Zohar holds that it is אסור בהנאה.
The Poskim rule, wherever there is a Machlokes between
Reb Shimon and R. Yehuda the Halacha is like R. Yehuda."
I find it also puzzling, if we assume the Bais Yosef held that the Zohar is from R. Shimon B.Y. How does he claim that we should follow R.Shimon's ruling in the Mishna because R. Shimon in Zohar also agrees.
It's like saying that he is right because he himself said so.
Perhaps we can explain, the Bais Yosef means to say, to be
Machmir because על פי קבלה (Zohar) it is אסור בהנאה.
According to this, we can answer the Rema's Kushya.
True Halachacially it should be Muter because we always
Pasken like R. Yehuda against R. Shimon. However, according
to **Kabbala (Zohar), it is אסור בהנאה like R. Shimon. אמבצי
*סתם ר' שמעון הוא רשב"י
** עיין מ"ב ס' כ"ה ס"ק מ"ב בשם הכנה"ג