to gather: to bring scattered people or things together into one place
The word names the act of collecting what is spread out and pulling it into one location. Its subjects range from a military commander mustering his troops, to a king assembling prophets before a campaign, to a man accumulating silver. The thing gathered can be people, livestock, grain, wealth, or even nations. What unifies every instance is the movement from scattered to collected, from many places to one.
The word is especially prominent in the prophets of the exile and return, where God is the subject and a scattered people is the object. In this use the word carries the full weight of Israel's hope: God will personally search out and pull together the people he has dispersed among the nations, and bring them home. This is not a different sense but the same gathering action applied to the defining event of Israel's history, displacement and restoration.
A further shade appears in contexts where people gather themselves: an army assembles, a nation comes together at a sacred site, enemies converge for battle. The reflexive and passive forms carry this nuance, where the focus shifts from the agent who calls the gathering to the crowd that forms. The same core meaning holds: what was apart is now together.