The reasons that your dog keeps gagging are probably because it is suffering from laryngeal paralysis and infections. Infections like Kennel cough are quite common among dogs. This is a respiratory infection and can make your dog gag. Otherwise, there is also a chance that your dog is trying to clear its throat, which probably has some grass, dirt, or some foreign material lodged inside it. Its attempt to bring out these substances is probably making it gag as well.