to grind, to calender, to polish, to roll with a stone roller
砑 centers on smoothing or shaping a surface by pressing and rubbing with a hard tool: grinding, polishing, calendering, or rolling with a stone roller.
砑 combines 石 (stone) with 牙, which likely contributes the sound. The stone radical suggests a stone tool used for grinding or polishing.
A stone 石 roller with tooth-like 牙 projections grinds and polishes the surface as it rolls.
For ガ, imagine the grinding stone making a harsh 'ga-ga-ga' sound as it rolls over the surface: ga -> ガ.
bladder moon shell (Glossaulax didyma)