Probable catalytic subunit of a GTPase activating protein that has specificity for Rab3 subfamily. Rab3 proteins are involved in regulated exocytosis of neurotransmitters and hormones. Specifically converts active Rab3-GTP to the inactive form Rab3-GDP (By similarity). The Rab3 GTPase-activating complex is a heterodimer composed of CG31935 and rab3-GAP/CG7061. Belongs to the Rab3-GAP catalytic subunit family.