| Abbreviation | Definition |
|---|---|
| GRAB | Galactic Radiation And Background |
| GRAND | Gamma Ray and Neutron Detector (NASA) |
| GRB | Gamma Ray Blast |
| GRB(s) | gamma ray burst(s) |
| GRO | Gamma Ray Observatory |
| GSC | Guide Star Catalog |
| GSMT | Giant Segmented Mirror Telescope |
| GSY | Galactic Standard Year |
| GT | Galaxy Tour |
| GTC | Gran Telescopio Canario |
| GTL | Geometric Time Leverage |
| GU | Galactic Unit |
| GV | Group Velocity |
| GXY | Galaxy |
| GY | Gigayear (1 billion years) |
| GYR | Gigayear (1 billion years) |
| HAC | Huachuca Astronomy Club |
| HAT | Highest Astronomical Tide |
| HAWAII | HgCdTe Astronomical Wide Area Infrared Imager |
| HB | Hot Band |