mountain saddle
乢 centers on a mountain saddle: the low point or pass between peaks. The meaning is specific to that geographical feature.
乢 combines 山 (mountain) and 乚 (a hook shape that may represent a bend or depression), suggesting a low point in a mountain range. The exact historical development is uncertain.
The left side is a mountain 山, and the right side looks like a bent hook 乚 dipping down. Picture a mountain range with a saddle-shaped dip between two peaks.
For ガイ, imagine a guy hiking through a mountain saddle: guy -> ガイ, and he rests at the low pass between the peaks.