rock
嵒 centers on rock as a solid, rugged natural formation. The meaning is straightforward: a rock or rocky place.
嵒 is composed of 品 (goods, items) over 山 (mountain), but the historical origin is uncertain. It may have originally depicted a rocky mountain with piled stones, but the exact development is unclear.
Picture a mountain 山 with three stacked mouths 品 at the top, like boulders piled high on a peak. Together they form a rocky outcrop: 嵒 is a rock.
For ガン, imagine a gun made of rock: a stone pistol that fires pebbles. Gun -> ガン, and the rocky weapon reminds you of the kanji's meaning.