Grey Knights are commonly teleported into position where they are needed most. Any unit of Grey Knight Terminators, a Grey Knight Hero or a Grey Knight Teleport Attack may be held in Reserve and enter play by Deep Strike using the Deep Strike scenario special rules (see page 132 of the Warhammer 40,000 rulebook) even in missions which do not normally allow Deep Strike or Reserves.
Errata — Chapter Approved: Codex: Daemonhunters Q&A (v1.0): If Allied Imperial Guard are taken, a vehicle with improved comms may be used to re-roll the Reserves roll for these teleport attacks (as well as for Orbital Strikes).