Why the range does not solve this
Ask a unit how it trains counter-drone gunnery and the answer is usually clays from a trap. That is genuinely useful for the swing and the lead, and it is nothing like the problem. A trap throws a fast, falling, predictable arc across a known line. A drone is slower, higher, holds a heading, stops, and changes its mind.
The alternative is to fly an expendable airframe and shoot it down. That teaches the right picture exactly once, and the cost of the lesson is a drone. Units that can afford it do it rarely; units that cannot do not do it at all, and go into the problem having never engaged a moving aerial target that behaves like one.
GARM sits between the two. The carrier flies a briefed profile at a briefed height and speed, so the picture is the one the gunner will actually see. The target is a steel silhouette that does not care how many times it is hit. The consumable is a clay pigeon.