the movement of a fish's mouth at the surface of the water
噞 describes the specific motion of a fish's mouth opening and closing at the water's surface, as if gasping or feeding.
噞 combines 口 (mouth) with 僉, which likely contributes the sound. The character specifically depicts a fish's mouth movement at the water surface.
The left side 口 is a mouth, and the right side 僉 looks like a person with a big hat, but here imagine a fish poking its mouth out of the water, opening and closing it rhythmically.
For ゲン, picture a fish at the surface making a gentle 'gen' sound as it opens its mouth: gen -> ゲン.