This gall was new to me, and this one as well.
I live in the UK where the Canadian goldenrod Solidago canadensis is now naturalized, it was introduced in the seventeenth century, and as far as I know there are no galls in it over here. The bees love it though.
I've just done a search for galls in our native goldenrod Solidago virgaurea and got zero hits, by the way, recently it has been badly upstaged by the Canadian goldenrod and Solidago gigantea.
Must look around from now on for galls in herbaceous plants, I only knew cases in shrubs and trees.
Back to this gall, I did open one see here. Apparently the larva hibernates in the gall, which turns brown in the winter, pupates in the spring and the emerges soon afterwards.
Patrick Coin has kept some galls through the winter and got very successful emergences, see his photos of an intact brown gall, an empty one, and the culprit, a fly.