A person wins a debate before an audience if he persuades audience members who did not agree with him beforehand that his position is correct to a degree that his opponent fails to meet. A person wins a debate absent an audience if he persuades his opponent that his position is correct. Please note that it is possible for a skilled debater to win a debate even when the position he holds is completely incorrect.
Gary, on Dan's blog, claims that he always wins debates againsts but they don't admit it. I'm going to go out on a limb here and say he hasn't been persuading audiences. This leaves him claiming that he wins by convincing his opponent without actually convincing his opponent. Yeah, it doesn't make any sense to me either.