Gnostes (γνώστης) means an expert or one who is thoroughly acquainted with something. It appears only once in the New Testament (Acts 26:3), where Paul addresses Agrippa as an expert in Jewish customs and controversies. The word emphasizes depth of acquired knowledge — expertise rather than just familiarity.
Paul's rhetorical strategy in Acts 26 is to appeal to Agrippa's expertise (gnostes) in Jewish matters as the basis for expecting a fair hearing. He is not flattering; he is stating a fact that makes his defense credible to this particular audience. This models the wisdom of knowing one's audience — the gospel must be communicated in ways that engage people where they actually are. Agrippa's response in 26:28 ('Almost you persuade me to be a Christian') is the closest any political authority comes to conversion in Acts. Deep knowledge without surrender is the tragedy of Agrippa.