Enhances GTPase-activating protein (GAP) activity of regulator of G protein signaling (RGS) proteins, hence involved in the termination of the signaling initiated by the G protein coupled receptors (GPCRs) by accelerating the GTP hydrolysis on the G-alpha subunits, thereby promoting their inactivation (Probable). Increases RGS9 GTPase-activating protein (GAP) activity, hence contributes to the deactivation of G protein signaling initiated by D(2) dopamine receptors (By similarity). Along with gnb5b, plays an important role in neuronal signaling, including in the parasympathetic, but not sympathetic, control of heart rate (PubMed:27523599). May interact with RGS9; this interaction stabilizes both proteins and increases RGS9 GTPase-activating protein (GAP) activity, hence accelerating the deactivation of D(2) dopamine receptor-mediated signaling. No visible phenotype; due to the redundancy with gnb5a. Simultaneous knockout of gnb5a and gnb5b results in no striking dysmorphologic features, but the larvae show impaired swimming activity, remain small, and generally die 7-14 days post fertilization (dpf), most likely as a result of their inability to feed. Belongs to the WD repeat G protein beta family.