Gustatory receptor which mediates acceptance or avoidance behavior, depending on its substrates. GPRgr22 and GPRgr24 together are sufficient for olfactory carbon dioxide-chemosensation. Carbon dioxide-responsive neurons coexpress GPRgr22 and GPRgr24 in the maxillary palp at both larval and adult life stages. Belongs to the insect chemoreceptor superfamily. Gustatory receptor (GR) family. Gr21a subfamily.