Plain English Meaning
GS in a weather report means small, soft ice pellets are falling — bigger than sleet but smaller and softer than true hail.
Full Definition
In a METAR, GS is the precipitation code reported when small hail or snow pellets (graupel) are falling at the station. Snow pellets are white, opaque ice particles, generally 2–5 mm in diameter, that are soft and crumble easily when they hit a hard surface. Small hail refers to ice particles in the same small size range. Both form when supercooled water droplets freeze onto falling snow crystals, building up a soft, rimed ice particle.