A GD, especially when they get old, instead of grinding things up, tears them up, and the particals are much harder to flush down the drain. There could be a bunch of crud stuck in there that you can't dislodge wtih the flushing. Unless you are considering buying the forclosure, I'd just put the plug in the drain when not actually draining anything, and forget about it. If the house can't supply the required hot water to bath, etc., it technically could be condemmed. Again, if you are moving out, probably not worth trying to pursue, since the landlord isn't going to do anything in the timeframe remaining.
If you need a reason to break the lease, that is one in most places.