Knowledge about the subject can never be replaced in a GD. No matter how good you might be communicating, but if your sentences don’t reflect that you are knowledge bank then it’s probably not going to work out.
You are required to talk in a GD but inputs that don’t contain any substantial value will not help in any way. One has to keep himself updated by knowing what all is happening around the globe.
In a GD you are required to carefully listen to the other person’s thoughts and keep an argument, example or a supportive statement, fact, example ready to participate in the discussion. This shows how alert you are, how much importance you give to when someone is putting his or her point forward.
This also shows how good listener you are, at time, things turn up to be a mess when you feel that what is being said is not making sense. Don’t get irritated. Remember that you need to be calm and composed. Many times after a mess happening in a GD, candidates who are calm are selected.