Catalyzes the stereospecific, NADPH-dependent reduction of L-glyceraldehyde 3-phosphate (L-GAP). The physiological role of gpr is the detoxification of L-GAP, which may be formed by non-enzymatic racemization of GAP. Also involved in the stress response as a methylglyoxal reductase which converts the toxic metabolite methylglyoxal to acetol in vitro and in vivo. Homotetramer. Cells lacking this gene show 23% decrease in the amount of acetol. Belongs to the shaker potassium channel beta subunit family.