bud (plant)
莟 centers on a plant bud: the small, unopened stage of a flower or leaf before it blooms.
莟 combines the plant radical 艹 with 含, which likely contributes the sound and the idea of something held inside, like a bud enclosing the future flower.
The grass top 艹 sits above 含, which suggests containing or holding in. Picture a plant holding its future flower tightly inside a small bud.
For ガン, imagine a bud that looks like a tiny gun barrel before it opens: gun -> ガン, and the bud is ready to fire open.