The gap is explained by differences in intelligence and effort.
Effort! Is anyone allowed to look at that any more? Somehow students who have poor attendance, pay little attention when present, do little work in class and almost no homework are expected to achieve similarly to students who do the opposite. That seldom happens.
The fact that high school GPA's have a much smaller gap between groups than SAT scores probably means that students in schools with relatively high GPA but low SAT grade much easier or teach much easier courses than schools where GPA and SAT are similar. The SAT is a test that is the same for all who take it. GPA can mean anything.