Chansey

Unfortunately Chansey is not a viable defender. Even though it has very high HP, its Defense stat is so low that it doesn't translate into as tanky Pokémon as one might think. A ridiculously low attack stat on top of that means that it will do barely any damage even with the optimal moveset. Chansey's defensive capability equals Raticate's with the worst moveset.

Hatched a Chansey when I was (roughly) level 20. I powered it up to CP 392 and put it in a gym, more as a "hey look what I've got" (live in a big town/small city in Scotland so rare pokemon are ultra rare) I tried to train it up but due to its tankiness of 321 hp I had nothing with a lower CP that could take it out effectively, I imagine now it may be easier with the 6 pokemon option but all 6 would have to be under 392 CP to get good prestige. It only lasted an hour in the gym so it was taken out easily by an attack. Would do it again though because like you I get bored of the usual suspects for gym defenders.

I've not seen one, but this website had a post a while ago stating that she's not a good defender. Her defense is sufficiently low that it greatly reduces the effectiveness of her huge HP. Then her attack is even lower that she's not a damage threat.

The big plus to a Chansey would be the amount of time. Your attacker won't be able to clear the whole gym because of the time needed for Chansey. EXCEPT they only have to beat Chansey once, then she's out of the bottom and they can start fresh and beat the whole gym each time. So she's basically useless. Unless gyms change to give the bottom pokemon some more safety (i.e. I've constantly been saying gyms should downgrade at different levels than they upgrade, should go from 10 to 9 at 42000 not at 50000) Chansey won't be worthwhile.