Gya and Ga, are symbols for units of time, specifically gigayears ago and gigaannum (or giga-annum), respectively. They represent 10⁹ (1,000,000,000) years. Ga is the base unit, and Gya is relative to the present. The G in both symbols (but not the g in giga-, unabbreviated) is conventionally capitalized. Both symbols are pronounced by sounding out their letters, g-a and g-y-a, or by giving their unabbreviated forms. The terms are often used in geology, paleontology, geophysics, and astronomy disciplines including celestial mechanics.

Gya or GYA may refer to:

• Gya, symbol for gigayears ago, a unit of time equal to one billion (short-scale, i.e. 1,000,000,000) years before present.
• Gya, symbol for the gray, a unit of radiation exposure, equal to 100 roentgen
• Gya, Ladakh, a village in India
• Global Young Academy, an association of early-career scientists
• Guayaramerín Airport, an airport in Bolivia
• Gulf Yachting Association, a boating organization on the United States coast along the Gulf of Mexico