A fighter with the Psyker keyword is a sanctioned channel to the warp, able to manifest the Wyrd Powers of any psychic discipline it has access to. Among the Grey Knights the keyword belongs to every living battle-brother — the Grand Master, the Librarian, the Champion, the Chaplain and Techmarine, and every Strike, Terminator, Paladin, Purifier, Interceptor and Purgation Knight — for there are no Grey Knights who are not psykers. The two Nemesis war-machines, the Dreadknight and the Venerable Dreadnought, do not carry it: they are weapons the brotherhood wields, not casters in their own right. Each Psyker manifests using the standard psyker rules, and every cast carries the warp's price. Sanctic powers are drawn free at creation — the gift is baked into the Grey Knight's cost — and the brotherhood loans its casting to no lesser thing.
Codex · Grey Knights