'highest knowledge', gnosis, refers to the perfect knowledge of the Saint (Arahat; s. ariya-puggala).
The following passage occurs frequently in the Suttas, when a monk indicates his attainment of Holiness (arahatta): "He makes known highest knowledge (aññam vyākaroti), thus: 'Rebirth has ceased, fulfilled is the holy life, the task is accomplished, and there is no more of this to come.' "
The 'faculty of highest knowledge' (aññ' indriya = aññā-indriya; s. indriya), however, is present in six of the eight stages of holiness, that is, beginning with the fruition of Stream-Winning (sotāpatti-phala) up to the path of Holiness (arahatta-magga).
See Dhs. (PTS) 362-364, 505, 553; Indriya Vibhanga; "Path" 162.