Probable regulatory subunit of a GTPase activating protein that has specificity for Rab3 subfamily. Rab3 proteins are involved in regulated exocytosis of neurotransmitters and hormones. Rab3 GTPase-activating complex 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. Belongs to the Rab3-GAP regulatory subunit family.