The horse radical on the left, in its compact simplified shape. It anchors the meaning: to gallop is an action of a horse. The same radical groups 驰, 驶, and 骋 as horse-motion verbs.
phonetic
甹pīng
phonetic component
Supplies the sound. The reading has drifted from pīng to chěng, so it now serves only as a pronunciation hint with no meaning contribution to this horse verb.