states some other flies do make similar galls.
Gall is relatively distinctive--see photo.
The female deposits an egg at the tip of the growing plant. Its larva secretes a chemical that prevents the goldenrod stem from growing although it continues to produce leaves, thus a shortened bunch of leaves is formed.
Arnett, p. 859, describes briefly (2)
Eastman, p. 154, illustrates gall. (1)