I think Grey Knights are a decent place, probably a solid B tier codex since the AoC balance update. With a good knowledge and a good list you should be able to be fairly competitive, but you're still gonna get smashed by the most competitive Eldar or Nid list - feel like that's just how it is!
Please take this advice with a pinch of

as I am not a grey knight player but this is what I would do in your situation:
Play heavily into the new AoC as it is powerful. Remember that dreadknights don't benefit from this, neither does anything equipped with a storm shield/relic shield, or any way to mitigate
AP.
Take ObSec - lots of things now have it and I would say with the low model count
GK suffer from, its important to make every model count!
Paladins? If I'm not missing something and AoC applies to them, they are probably better than Custodes at the moment!
Anything with terminator armour is now far harder to kill - obviously it depends on your playstyle and what models you have, but I think they are good.