A gateway apparatus for allowing a plurality of apparatuses on a first network to co-operate with each other includes: a message monitoring unit which monitors messages communicated on the first network, and detects an apparatus discovery message for discovering an apparatus that satisfies an arbitrary requirement; an activation state managing unit which checks whether at least one target apparatus among the apparatuses that satisfies the requirement indicated by the apparatus discovery message is in an activated state for communication on the first network; a predicted activation time determining unit which determines a predicted activation time predicted as required to activate the at least one target apparatus; and a proxy response unit which transmits, in proxy of the at least one target apparatus, a response message including information indicating the predicted activation time when the at least one target apparatus is not in an activated state.