Is a gentile prohibited from castrating a human/animal as part of the Nohadite laws?
This matter is disputed in the Talmud and Poskim. Some Poskim rule that Gentiles are not prohibited against castrating. Other Poskim rule that gentiles are prohibited against castrating. Practically, they are to be stringent and not do so.
Asking a gentile to castrate an animal:
 Rav Chidka and Tana Divei Menashe in Sanhedrin 56b and in Bava Metzia 90a rules they are commanded against castrating; The Tana Kama in Sanhedrin ibid rules they are not included in the prohibition
 Rambam; Rosh; Rashba; Hagahos Maimanis, brought in Beis Shmuel 5/16
 Semag; Hagahos Ashri, brought in Beis Shmuel 5/16
 Beis Shmuel 5/16
 Michaber E.H. 5/14
 Literally “Kutti”
 The reason: This is forbidden due to Amira Lenachri, as every matter in the Torah that is forbidden for a Jew to perform is likewise forbidden for a Jew to ask a Gentile to do for him. [Beis Shmuel 5/16; Beir Hagoleh; Admur 243/1; 343/5; 450/20; Ribis 72; Sheila Usechirus 29; Michaber C.M. 338/6 [regarding muzzling animal]; Question brought in Bava Metzia 90a and above Poskim rule stringently-see Biur Hagr”a 5/32] Alternatively, the reason is because it is Biblically forbidden for even a gentile to destroy the reproductive organs of a human/animal, and it is part of the Nohadite laws. [Beis Shmuel 5/16; Rav Chidka in Bava Metzia 90a and Sannhedrin 56b]