Rory Gilmore graduated from Chilton with a 4.2 GPA.
Rory attends Chilton, a demanding fictional preparatory school, after leaving Stars Hollow High. Her academic performance is a major part of her characterization: she is a serious reader, competes intensely with Paris Geller, and aims for admission to prestigious universities.
The 4.2 figure is associated with Rory's graduation record, along with her status as Chilton's valedictorian. A GPA above 4.0 reflects the use of weighted grading, in which advanced or honors courses can receive additional value. It does not mean that a normal class was graded above the standard scale.
Rory ultimately chooses Yale University over Harvard, her long-standing dream school. Her academic success continues at Yale, where she studies English and writes for the Yale Daily News. The GPA is therefore one concrete measure of the scholastic achievement repeatedly emphasized in the series.