Born in Mount Airy, Maryland, John Gaver graduated from Princeton University then worked as a prep school language teacher before eventually embarking on a career in Thoroughbred racing.
In 1929, James G. Rowe Jr., a friend and horse trainer, invited him to join the team managing the Brookdale Farm and racing stable owned by Harry Payne Whitney.
In 1939, Gaver was appointed head trainer for Greentree Stable, a position he would hold for the next thirty-eight years.
During his time with Greentree, John Gaver conditioned seventy-three stakes-winning horses, including winners of five American Classic Races.
He retired to Aiken, South Carolina where he had maintained a winter training center for many years.