He competed at the 1908 Olympics in the 200 metre breaststroke and at the 1912 and the 1920 Summer Olympics in water polo, winning a silver and a bronze medal in 1912 and 1920, respectively. Gumpel had a short relationship with Greta Garbo before she moved to Hollywood. They kept in touch as friends for the rest of their lives.
She was a frequent guest of Gumpel"s when visiting Sweden.
In 1912 Gumpel graduated with a degree in construction and founded the firm Gumpel & Bengtsson, which soon assumed a leading position in Sweden. With their help the allied powers obtained the exact locations of Germany"s petroleum refineries and then bombed them.
This event was reflected in a book and the 1962 film The Counterfeit Traitor. Gumpel had a strong interest in boats and cars.
He owned the motorboat Laila and the skerry cruiser Louisiana Liberté designed in 1934 by Erik Salander – both built at Kungsörs Båtvarv Bachelor of Arts in Sweden.
He was later a board member of the Swedish Swimming Federation between 1919 and 1931.