I may be a little biased as I am not a big fan of seasons 12 or 13 anyway so take this with a pinch of salt but the 6 I have given is on the generous side.
As with most of season 13, this AGAIN is not about Sam and Dean, it's about them watching other story lines and participating just to...well not sure quite what they are there for. A petty revenge story by Gabriel who they really shouldn't have bought back. This episode quickly introduces characters and then destroys them before you really know or realise who they actually are, what they can do or what they are supposed to have done. I feel like the Gabriel jokes are old and forced and the banter tiring. Not sure about the lazy fight scene, with Dean, Sam and Gabriel and some norse gods I expected better than the lights out scene! What a let down.
The story about the alternate universe is boring and predictable with very few scenes, and those we get are cheap and basic. Kevin's one liners border on pathetic and are blatantly recycled from other episodes. Mary really isn't working in this episode either as a leader or a mentor. Her protective maternal relationship with Jack just doesn't work, it is disjointed and out of place especially considering she couldn't even make Dean and Sam her boys in the first place.
Here's to the undying hope that the next episode will be better, which I am sure it will. This is Supernatural after all.