This is a good question and since I've been dealing with all the children of Eve texts lately, I have had to re-evaluate this common knowledge. Yes, the texts say that the Sethians thought they were the carefully preserved race of Seth, that they even had a heavenly pre-existence. There were other races descended from Eve's other children, but the seed of Seth was elect.
Now this sounds elitist and exclusive at first glance. Until you ask the question, how do you become a Sethian? The answer is that you convert. In other words the Sethians, like the Valentinians, thought that your response to the religious message - i.e. you choose to become one of us - is what signaled that you were an elect. Certainly the Valentinians at least were concerned about proper sex so that an elect might be conceived, but one only knew if the child was elect if the child responded instantly in a positive way to the Valentinian message.
I haven't figured out how the Mandaean (modern Gnostics) notion that only children born to Mandaean fathers and mothers could be Mandaean, but I imagine that this was a later development and had to do with social issues. As the community moved and settled into new locations, intermarriage was probably a natural response to keep the faith "pure".
At any rate, the ancient Gnostic Christians look to be every bit as much like any other Christian tradition at the time. If you converted, you became part of the saved. If not, you were doomed. They were no more elitist or exclusive in terms of salvation than the standard Christians in that period (or many Christians today).