GY, Gy, or gy may refer to:

• Gray (unit) (Gy), SI unit of absorbed radiation
• Giga-year, 1,000,000,000 years (non-SI)
• Galactic year, the time it takes for the Solar System to orbit the Galactic Center

• Gabon Airlines (IATA:GY)
• GenCorp (NYSE:GY)
• Green Youth (disambiguation), a Green party youth wing

• Hungarian gy, an alphabetic digraph
• gy, a digraph in the Tibetan pinyin transliteration system

• Pierre Gy (1924–2015), French chemist and statistician
• Garin de Gy, 14th century Catholic Master of the Order of Preachers

• GY postcode area, of the Bailiwick of Guernsey
• Great Yarmouth
• Grimsby

• Guyana (ISO 3166: GY)
• Gy, Haute-Saône, France
• Gy, Switzerland

• .gy, the country code top level domain (ccTLD) for Guyana
• Mazda GY, a car engine
• Grapevine yellows, a plant disease
• Gy, the on-line charging interface in the GPRS core network