galerio

One root, no seam. Galerio looks as though it might be gal- plus something, and it is not. There is no -eri- suffix in Esperanto: the official list runs -ar-, -ej-, -il-, -uj- and the rest, and nothing in it makes -erio. Compare French boulangerie and English bakery, where a real suffix is doing real work; Esperanto builds that with -ej- instead and would say bakejo. Galerio came over whole.

Four senses, and the order matters. It started as a covered walk, which is sense one: a long passage with one side open, the kind that runs along the front of a big house. Because people hung pictures along it, it became the room you look at art in, which is sense two and the one you will meet most. Sense three is a mine tunnel. Sense four is the highest balcony in a theatre, the cheap seats, what English calls the gods and French le poulailler, the henhouse.

Not a muzeo. A museum holds a collection and keeps it; a gallery shows work, often work that is for sale. The distinction is the same one English makes and Esperanto did not have to invent it. An ekspozicio at rank 2,298 is the event rather than the room.