the mouth of a fish at the surface of the water, gasping for breath
喁 centers on a fish's mouth breaking the water surface, often with a sense of struggling for air. The gasping meaning extends naturally from that image of a fish opening its mouth at the surface.
喁 combines 口 (mouth) with 禺, which likely contributes the sound. The character is associated with the image of a fish mouth at the water surface, but the exact historical development is uncertain.
The left side 口 is a mouth, and the right side 禺 looks like a fish with its head tilted up. Picture a fish pushing its mouth above the water, gasping for air.
For グ, imagine a fish going 'goo' as it gasps at the surface: goo -> グ, and the mouth opens wide.