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 (By similarity). Homotetramer. Belongs to the shaker potassium channel beta subunit family.