Gustatory receptor which mediates acceptance or avoidance behavior, depending on its substrates. Required for the response to N,N-Diethyl-meta-toluamide (DEET), the most widely used insect repellent worldwide. Functions as a pheromone receptor for a male inhibitory pheromone and promotes male-male aggression and suppresses male-male courtship. Also promotes preferentially virgin females courting over mated females. Expressed in the adult labellar chemosensory neurons. Expressed in tarsal neurons for male-male courtship suppression. In larvae, is expressed in neurons of the terminal external chemosensory organ, and the dorsal and posterior external chemosensory organs. Impairs avoiding N,N-Diethyl-meta-toluamide (DEET). Leads to high male courtship toward males and mated females. Also leads to diminished aggression level of males. Belongs to the insect chemoreceptor superfamily. Gustatory receptor (GR) family. Gr21a subfamily.