I've been having this issue for the past hour. I'm on ps5, and can't progress further in act three because I can't travel to the lower city after defeating Gortash. A reddit fix suggested taking his body and putting it in your treasure chest in camp, didn't work for me I didn't have the option to do so, but thought I'd share it here and see if it works for others.
Doing that to Gortash's body won't do anything - if you can't travel to the Lower City it means the corrupted NPC body is in the Lower City somewhere. You'd have to go back to a save there and find it - it'll be the body of a creature you've killed that's stuck on greater than 0hp.
To be able to pick up a body you need sufficient carrying capacity - it needs to be a strong character who's not already carrying too much. But some people have said that using AOE spells to reduce the body to 0hp also works, and that might be simpler.
