The gemarah tells us from a ribui of Ish Ish that Goyim are Muzhar in Arayos like a Jew. The Netziv in Meromei Sodeah says that this cannot mean they have the same amount of aryos and Jews since that is not true. He therefore, establishes the Gemrah to mean they get Kares like Jews.
The Sha'areim Metzyanim B"halachan written by Rav Braun on this Gemrah says Tzarich Iyun Gadol since we see clearly from the Gm' in Sanhedrin that they do not get kares, rather they get Mitah B'sayif. Perhaps, the Rav can suggest a Teretz or direct me to a given Teretz. I have already checked all the mephorshim in back of the GM' as well as the Turei Even.
The Gemara in Sanhedrin (57b) writes clearly that the Drasha of Ish Ish is either for (a) Arayos Yisrael (which the Gemarah writes is Na'arah ha'Me'orasah) and all the other Arayos are learned from Laymor, or (b) to teach the Arayos which are Kares for Yisrael. Rashi writes that they (Akum) will then have Sayif.
No matter what, they do not have paternal Arayos, and according to some Rishonim neither do they have Arayos which come through marriage. The NeTZiV is a Mitzvah l'Yashev.
Dov Zupnik