Goldfish breathing at surface! :(?

probably not oxygen deficiency it could be ammonia or nitrite poisoning. They will go to the surface as the ammonia and nitrite can be damaging to their gills. Brand new filter means no bacteria which is bad for the fish do a water change of 50% and probably should get a test kit or just make that fish a starter fish. Its cruel but some people dont want to spend that much money on a testing kit. A good 3-4 weeks and the tank should be cycled just dont throw away the filter bag but wash it in old tank water. But for now water changes every few days