1) HALACHAH: MAY ONE DERIVE BENEFIT FROM THE "GID HA'NASHEH"
OPINIONS: The Gemara cites an argument between Tana'im whether or not one may derive benefit from the Gid ha'Nasheh, which the Torah prohibits one from eating. Rebbi Shimon maintains that one may not derive benefit from it, just as one may not eat it. He asserts that the Gid ha'Nasheh does not have the taste of meat and is therefore not included in the verse that permits one to derive benefit from the edible parts of a Neveilah. Rebbi Yehudah maintains that the Gid ha'Nasheh does have the taste of meat, and therefore it is included in the verse that permits one to derive benefit from the edible parts of a Neveilah.
What is the Halachah with regard to these two matters of dispute? Does the Gid ha'Nasheh have the taste of meat or not, and may one derive benefit from the Gid ha'Nasheh (and, for example, sell it to a Nochri)?
(a) The ISUR V'HETER HA'ARUCH (21:4) asserts that the Gid ha'Nasheh does have the flavor of meat, and therefore one is permitted to derive benefit from it. He ascribes this opinion to the RAMBAM and ROSH.
(b) However, the Acharonim point out that both the RAMBAM (Hilchos Ma'achalos Asuros 8:5) and the ROSH (Chulin 7:17) rule that the Gid ha'Nasheh has no taste. Nevertheless, the Rambam (ibid. 8:14) and the Rosh (ibid.) both rule that one is permitted to derive benefit from the Gid ha'Nasheh. (That is, they are lenient in both respects.) Similarly, the TUR (YD 65:9) concludes that the Gid ha'Nasheh has no taste, and yet it is permitted b'Hana'ah.
The Rosh explains that although the Gemara here links the two matters, it does so only according to the opinion of Rebbi Avahu. According to Chizkiyah, the two matters are unrelated. Since the Halachah follows Chizkiyah (as the Rosh asserts), in practice the two matters are unrelated.
The MAGID MISHNEH, in the name of the RAMBAN, suggests further that it is possible that the Rambam and Rosh rule leniently even according to Rebbi Avahu and permit one to derive benefit from the Gid ha'Nasheh for other reasons (such as the Kal v'Chomer from Chelev; see Pesachim 23b).
(c) TOSFOS in Chulin (99b, DH v'Hilchesa) rules that the Gid ha'Nasheh has no taste, and therefore it follows that one may not derive benefit from it. This is the ruling of RABEINU YONAH (cited by the Rosh) and other Rishonim.
HALACHAH: The BEIS YOSEF (YD 65) first cites the Rishonim who permit one to derive benefit from the Gid ha'Nasheh. He then quotes the opinion of the Zohar that states that the Gid ha'Nasheh is Asur b'Hana'ah. He concludes that one should be stringent and follow the opinion of the Zohar. Apparently, the Beis Yosef means that based on reasons of Kabalah, one should conduct himself more stringently than the Torah requires and refrain from deriving benefit from the Gid ha'Nasheh.
The DARCHEI MOSHE argues with the Beis Yosef's ruling. The author of the Zohar is none other than Rebbi Shimon (bar Yochai), whom the Gemara quotes as the Tana who maintains that the Torah prohibits deriving benefit from the Gid ha'Nasheh. Since the Halachah follows the view of Rebbi Yehudah who argues with Rebbi Shimon in this matter, the words of the Zohar should have no bearing on the Halachah in this case.
The SHULCHAN ARUCH (YD 65:10) indeed rules leniently, like the Rosh (b).