To declare something to be untrue or invalid.

अविश्वास करना

English Usage: She did not gainsay his claims during the meeting.

Hindi Usage: उसने बैठक के दौरान उसके दावों का अविश्वास नहीं किया।

The act of denying or contradicting something.

अस्वीकार करना

English Usage: His gainsaying of the facts shocked the audience.

Hindi Usage: तथ्यों का उसका अस्वीकार करना दर्शकों को चौंका दिया।