The validity of the grade point average
The grade point average could be a valid measure of a student's overall level of academic achievement only insofar as the individual course grades on which the average is based are valid measures of academic achievement within those particular courses. So the first question to ask is whether course grades are valid measures of the variable in question when considered individually.
As a grade-giving member of the professoriate, I would of course despair at the thought that course grades are wholly lacking in validity. Still, it seems entirely clear to me, and I expect it does to you as well, that individual course grades are not perfect measures of academic achievement in each and every instance. Sometimes they underestimate it, and sometimes they overestimate it. Then, too, there is the fact that the relationship between grades and academic achievement can be quite different from one institution to another, from one course to another, and even from one section of a course to another. Not to put too fine a point on it, a B+ from Professor Layedback can mark about the same level of academic achievement as a C+ from Professor Toughgut; a B+ in a course on wine tasting can mean something quite different from a B+ in a course on biochemistry; and a B+ average at Suds 'n Surf U. is not at all the same thing as a B+ average at the College of Academic Rigor.
The bottom line is that when you take the average of course grades as a uniform measure of students' overall levels of academic achievement, the validity of that measure is problematical and equivocal, to say the least.