OP, great thread. I've actually been thinking about something similar lately, for whatever reason. My boss likes to gossip and tell me stuff about people at work that I don't even really know... I don't like it much, especially when she interrupts me doing actual work, but she seems to find it valuable to "get things off her chest." When it's me and my friends, I kind of go by instinct, which I know isn't very helpful for anyone else--I start out telling news about people we both know, and if we go on for too long and take a certain "tone," I start to feel uncomfortable and like we're getting into (bad) gossip, so I stop.
I think a certain amount of commiserating about a mutual, irritating acquaintance can be helpful, though. Once a few years ago I had a newish co-worker who was rather annoying--he was an arrogant know-it-all, basically, though underlying that a decent guy. I remember that several of us--not him--were in a car going somewhere, and someone just said, "You know, I find Mike a little annoying sometimes," and the whole car just sort of sighed with collective relief as we all started talking about the ways he'd irritated us. We'd all been suffering in silence, feeling like maybe it was only us and not really him. I'm sure some of our subsequent conversations about him did venture into (bad) gossip, but others were really helpful in blowing off steam and giving each other new ideas for how to deal with him.
Regarding celebrity gossip... Although I don't believe they ought to be subjected to paparazzi and gossip just because they've chosen a public profession, unfortunately you pretty much have to expect it these days. I think the People magazine-level "human interest" stuff is more or less fine--stuff that's positive like romance and babies or even overcoming adversity. I don't like the catty websites that exist solely to mock and belittle celebrities, call them names, make up nasty stuff based on the merest hints, etc.. And even on the more respectable (IMO) sites, when I start to read the comments I often start to feel icky, because you see these people who just want to find flaws with everyone--if they're single that means they're in the closet, if they're dating they must be cheating, if they've lost weight it must be due to drugs, if they don't go out much it's because their SO is isolating them. I guess that's the schadenfreude part, taking too much interest and delight in other people's misery, to the point of imagining misery for them if none can be proven.