Born in 1556 as the eldest son of Gamo Katahide, lord of Hino Castle in Omi Province. His childhood name was Tsuruchiyo. Sent as a hostage to Oda Nobunaga at age 13, Nobunaga recognized his brilliance and gave him his own daughter, Fuyuhime, as a bride. He compiled an impressive war record under Nobunaga. Around 1585 he received baptism from the Jesuit missionary Organtino and took the Christian name 'Leon.' After the Honnoji Incident he served Hideyoshi, and for his merit in the Odawara campaign of 1590 was granted Aizu Kurokawa (later Wakamatsu) — eventually ruling Aizu with 920,000 koku. At Aizu he undertook a major renovation of Tsuruga Castle (Wakamatsu Castle) and developed the castle town. He was also counted among the 'Seven Sages of Rikyu' as a leading disciple of the tea master Sen no Rikyu, demonstrating first-rate cultural accomplishment. In 1593 he fell ill at Nagoya Castle, and on February 7, 1595, he died at the Gamo residence in Fushimi at the age of 40. His cause of death is estimated to have been rectal or liver cancer, though later generations whispered of poisoning by Hideyoshi or Ishida Mitsunari.

A rare warrior who embodied excellence in both arms and letters. Nobunaga judged him 'no ordinary man,' and he excelled in martial valor, political skill, and cultural refinement alike. Devout in his Christian faith, he showed deep benevolence toward his retainers and subjects.

Revered locally as the wise lord who laid the foundations of Aizu-Wakamatsu's townscape. Having served both Nobunaga and Hideyoshi yet dying at 40, he is mourned as 'the man who might have become master of the realm.' A Sengoku prodigy with three faces: Christian lord, tea master, and warrior.