I find it rather strange how in one scene 8 balls run when they see an Archangel, but in another attack. They say how powerful Michael is, but it doesn't seem that way in battles. It feels like the Archangels had their "wings clipped" so to speak, so they weren't too powerful. I would expect an archangel to have powers somewhere mid way between God and man. The whole power structure sucked.
Oh, and why could Gabriel beat up Michael????
It would stand to reason that some of the 8 balls are willing to fight one, but others are too afraid. Like I'd crap my pants if someone pulled a butter knife on me.
Granted they can fly and know kung fu, there hasn't been much in the way of superpowers presented on the show except maybe Gabe being able to mind control lesser angels. People's perception of Michael's power is most likely simply a remnant of what he once was. The dood is a fallen angel now.
Speculating based on most stories of this genre, Angelic powers come from a connection with heaven. Heaven probably isn't really heaven anymore given god's abandonment of his creations. So that's my guess as to why archangels aren't very mighty.
As for Gabe vs Mikey? You should stop trying to size angels up based on tiers and legends depicted off screen in classical scripture. Yes, obviously this show uses the bible for most of its foundation, but of course it deviates greatly. The strongest fighter does not always win or else Ali would have never lost a boxing round. If you're looking for a more mystical answer, we could speculate that the fact Gabe has devout followers gives him a boost like how we find out Lucifer has been feeding on the sacrifice of humans.
I wish I had some more concrete answers for you, but speculation is 3/4'ths of fandom.