Why does gfr test ask about african-american? what's optimal gfr? are age, gender, and race taken into account? why?

It's the muscle mass: The muscle mass is more in african americans vs others, men vs women, young vs old. When muscle mass is high, creatinine will be expected to be high without actual kidney weakness (overestimates kidney weakness). Accordingly, we need to factor these elements in the equation to estimate kidney function (gfr).

Muscularity: The current practice of estimating GFR based on a spot serum creatinine in people who appear perfectly healthy includes an adjustment for muscularity based not on looking at the person's muscularity, but on the checkbox that indicates race. If this seems foolish to you, you are not alone -- sadly, the focus is on "tech" rather than physical exams & common sense in our dysfunctional system.