You're comparing apples to oranges here. Yes, full might of Oryx vs the full might of Gaul is a hilariously mismatched fight in favor of Oryx. Oryx is the biggest bad that we're likely to see from a 'mortal' enemy until someone else claims the mantle of Taken King. That doesn't mean Gaul isn't a massive threat to us, and how they act varies. Oryx only showed up because the death of Crota was such a massive dent in his tithe he couldn't ignore it. He had to compensate to keep his Worm fed so he didn't just bum rush the City like Gaul. We also had step by step instructions on how to use the same powers keeping Oryx strong and capitalize on his weaknesses thanks to Toland. Gaul on the other hand had no limitations and no instruction guide on how to beat him. He hit first and he hit hard, scattering the only source of organized resistance immediately. Does he have space magic yet? No, but we're so small by comparison it doesn't matter that we do, he has overwhelming firepower. If you know anything about Warhammer 40K, think of it like this. The City are like the Tau. We're pretty advanced, we got some cool stuff, but we're ultimately an insignificant blip compared to other powers in the universe at large. Oryx was like a Tyranid splinter fleet. Sure the Tyranids at whole are one of the most dangerous things out there, but we only beat a small group of them but we still feel like hit shit for it. Gaul and the Cabal are like the Imperium. Are the Tyranids at large more powerful? Probably, but we're so small in comparison to both it doesn't matter, the Imperium still crushes us like bugs if they care to take the time to do so.