Dr. Gray has conducted diverse epidemiological studies of infectious diseases for 25 years in 5 continents. He has authored more than 360 peer-reviewed manuscripts and book chapters. His published works have been cited by others more than 11,000 times (H-index 70 by Google Scholar).

Much of his work has involved identifying risk factors for occupational diseases, particularly for respiratory virus infections. He has studied numerous occupational groups including farmers, animal breeders, veterinarians, military personnel, turkey workers, poultry workers, horse workers, hunters, and pig workers. A strong supporter for the One Health approach, he has won multiple One Health research and training grants, helped to established centers of One Health (USA, Romania, China) and developed four graduate programs in One Health (PhD, MHS, and certificates).