The kilogram the SI standard. It's used in kilogram-meter-second reasoning systems as the base of derivative units, such as Newtons (1kg * G) or torque (N*m). (Basing these off of the gram would be... weird. You might protest the gram's the wrong size, but it's too late to change it.) Prefixes are applied as normal to 'gram' despite this (so milligram, microgram, and exotically gigagram) are correct.
Gigagram just seems exotic in the same way that megameter does - it's perfectly correct and understandable, it's just rarely necessary.
Gigagram just seems exotic in the same way that megameter does - it's perfectly correct and understandable, it's just rarely necessary.