The same tikkun as any other pre-marital or extra marital sex. Sleeping with a gentile is not a biblical prohibition, it's a rabbinic one.
Because if the following interpretation of the verse is true:
(Deut. 7:3): "You shall not marry them (the gentiles, about which the
Bible speaks in the previous verses), you shall not give your daughter
to their son and you shall not take his daughter for your son.
then this verse means to not marry any gentile, but this also means that you are obligated to kill every gentile you meet. Why? Because the previous verse says the following:
(Deut 7:2): " and when the LORD thy God shall deliver them up before thee, and thou shalt smite them; then thou shalt utterly destroy them; thou shalt make no covenant with them, nor show mercy unto them;"
You cannot cut this verse both ways. You can't say it's talking about inter-marrying with any and all gentiles, and then switch and say the commandment to kill these same gentiles doesn't apply in the same way. The only way to interpret this with any kind of sense is to take it in the proper context, which would be Deut 7:1
(Deut 7:1): "When the LORD thy God shall bring thee into the land whither thou goest to possess it, and shall cast out many nations before thee, the Hittite, and the Girgashite, and the Amorite, and the Canaanite, and the Perizzite, and the Hivite, and the Jebusite, seven nations greater and mightier than thou;"
You are not allowed to intermarry with the specific 7 Canaanite nations. So if by the off chance you happened to sleep with a Jebusite then yeah that would be a pretty grievous sin, almost as grievous as not murdering her [sarcasm].