Gilbert was educated at the Tualatin Academy and attended Pacific University in nearby Forest Grove, Oregon, where he was a member of the Gamma Sigma Fraternity. He left Pacific after 1902 and transferred to Yale University, financing his education by working as a magician, and earning a degree in medicine. His thesis had the title The Genito-Urinary Phenomena of Athletes.

An accomplished athlete, he broke the world record for consecutive chin-ups (39) in 1900 and distance record for running long dive in 1902. He invented the pole vault box and set two world records in the pole vault including a record for 12′ 3″ (3.66 meters) at the Spring meet of the Irish American Athletic Club, held at Celtic Park, New York, in 1906. He tied for gold with fellow American Edward Cook at the 1908 Summer Olympics in London for pole vaulting.