I believe the lack of a gasket doesn't mean it won't seal. In fact, if it required a gasket and it was missing the entire union would be pouring out water, not just a single stream. When you removed the union nut did you notice any cracks on the male threads on the pump side? The stream of water in the pic looks like a small crack has occurred under the union nut causing that thin spray. Before turning on your pump, remove the nut again and carefully examine the threads on both halves of the union. Because it is a black PVC it may very difficult to detect a hairline crack. If you cannot locate any cracks, reinstall the union and turn on the pump. If the spray is in the exact same location, I'll bet you have a hairline crack or damaged threads on the pump side of the union. Unfortunately, I don't know of a good way to permanently repair that!