Good question. In my case, I talked to the person in admissions who was making the decision and explained the situation and why I thought they should disregard the earlier grades.
But absent that conversation and direct evidence, GPA is a start. Assuming that the high school curriculum is at least passably similar to what they're going to expect from you in college (of course it isn't, exactly, college tends to be more heavily focused on tests than homework), a good GPA demonstrates that you will do the academic work required to succeed in class.
