The gemara concludes that a woman or child who don't have believability as an עד אחד נאמן באיסורין are nonetheless believed מסיח לפי תומו by something which is only d'rabonon (such as the bees nesting in someone elses yard, they are believed to say where it came from since the entire kinyan on these type of things is only d'rabonon. But by an issur d'oraysa we wouldn't believe a woman or child and certainly not a goy who is מסיח לפי תומו, with the exception of eidus isha that her husband died and by shevuya where chazal were especially lenient.
The Shulchan Aruch (98:1) rules that a goy is believed מסיח לפי תומו to say that he doesn't taste the issur that fell into kosher food, to be matir the food. The Shach and Taz both ask that the concept of מין בשאינו מינו בנותן טעם, that when food falls into a pot that has a different type of food, there is an issur d'oraysa to eat from the pot so long as the issur can be tasted (which in the absence of believing the goy we would assume that the taste in only nullified in 60 times as much). How can we believe a goy on an issur d'oraysa?
The Shach at first suggests that when there is a chezkas issur we don't believe a goy on an issur d'oraysa but when there isn't any chezkas issur we would believe a goy even on an issur d'oraysa. He then asks that if this is true the gemara should have no question from teruma and shevuya as to why we believe מסיח לפי תומו since there isn't any chezkas issur. Therefore the Shach concludes that since after the goy tells us that he doesn't taste the issur we will be tasting it and be able to call his bluff, he is more careful not to lie, so we can trust him. But in a situation where we won't be able to call his bluff we don't trust a goy even מסיח לפי תומו on an issur d'oraysa even if there isn't any chezkas issur.
The Taz offers an alternate approach. The din that a goy is not believed מסיח לפי תומו on an issur d'oraysa only applies to situations that require eidus, but in the realm of issur v'heter which doesn't require a real eidus, we can trust a goy מסיח לפי תומו. The Chavos Da'as (98:1) explains that the Taz doesn't mean to say that whenever an individual witness is believed it doesn't qualify as eidus, and therefore a goy is believed מסיח לפי תומו even by an issur d'oraysa. Because if that were true the gemara should not have any question from teruma and shevuya as to why מסיח לפי תומו since by those cases an individual witness is believed, so מסיח לפי תומו should also be believed. Rather the Taz acknowledges that even cases where an eid echad is believed would sometimes qualify as a real "eidus" and מסיח לפי תומו would not work on an issur d'oraysa. The litmus test as to whether something requires real "eidus" is whether or not there needs to be a haggada in beis din. Both the case of a kohen being mutar b'teruma and the case of shevuya require a hagadda in beis din therefore מסיח לפי תומו is not believed. But by issur v'heter which doesn't require a haggada in beis din,a goy מסיח לפי תומו is believed even by an issur d'oraysa.