For combining information in such a way you would have to believe that the average black with a IQ of 120 will get a higher GPA score than the average white person.

I think there little reason to believe that's true.

Could you explain your reasoning here?

IQ is a strong predictor of academic performance, and a 1.5 sd gap is a fairly significant difference. The only thing I could think of to counterbalance it so that the average white would get a higher GPA would be through fairly severe racial biases in grading policies in their favor, which seems at odds with the legally-enforced racial biases in admissions / graduation operating in the opposite direction. Not to mention that black African immigrants, legal ones anyway, seem to be the prototype of high-IQ blacks who outperform average whites.

I am a little puzzled by the claim, which leads me to believe I've misunderstood you somehow or overlooked something fairly important.