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.
Another unit identical to Gya is bya (for "billion years ago"), which may be in more widespread use. But some scientists prefer Gya because it uses the SI prefix multipliers giga- and thus avoids confusion over whether the short-scale (American and increasingly internationally standardized) billion, of 109 (1,000,000,000), or long-scale (former British, now mostly western continental European) billion, of 1012 (1,000,000,000,000) is meant, as well as avoiding preference for one of the two definitions of billion over the other.