valley
壑 centers on a deep natural depression in the land: a valley, ravine, or gully carved by water.
壑 combines 㕡 (a deep pit or channel) with 土 (earth), reinforcing the idea of a hollow in the ground. The exact historical development of the upper component is uncertain.
The top part 㕡 looks like a deep cut or channel, and 土 is earth. Picture a deep valley carved into the earth by a river.
For ガク, imagine a deep valley echoing with the sound of a gong: gong -> ガク, and the sound reverberates through the ravine.
ravine; chasm