wild goose
鴈 centers on a wild goose, a migratory waterbird. The meaning is specific to this bird and does not extend to broader abstract ideas.
鴈 combines 厂 (cliff) with 鳥 (bird), likely depicting a wild goose near a cliff or in flight. The exact historical development is uncertain, but the components suggest a bird associated with rocky or elevated places.
Under a cliff 厂, a bird 鳥 stands: a wild goose resting by the rocks.
For ガン, imagine a wild goose honking a loud 'GAN!' as it flies over the cliff: GAN -> ガン.