Yi Zhai, first, keep in mind that Vocabulary is no longer part of the official test. We deliberately left it in the study package because many students said they are helpful in increasing their word knowledge. Ok, what the solution means by "gainsay" being negative is that it means to contradict or to deny. It means "saying no to something". It doesn't matter if the thing being contradicted is bad or good. "Negative" here does not have a value judgment.