One can only assume that the Peverells were also descendants of Slytherin. Or perhaps that the Gaunts were descended from the Peverells who were descended from the Slytherins - after all, there must have been quite a few changes of name along the way. Just look at the English royal family, which has a direct line of descent down a similar time-period: from the Kings of Wessex to the Normans to the Angevins to the Plantagenets to the Lancastrians to the Yorks to the Tudors to the Stewarts to the Hanovers to the Windsors...I suspect Slytherin's descendants have gone through similar convolutions. After all, they can't ALL have married their cousins or they would have run out of cousins.