Gray Jacks are decrepit Hybrids that have almost outgrown their bodies. They are not an individual strain of Chimera, since they are simply elderly Hybrids.
In Resistance: Fall of Man, the Gray Jack's description implies that Hybrids grow as they age, their limbs gradually elongating. This increased size places tremendous strain on their cooling units over time, which are not designed to cool them at this size, and their high metabolic rate begins to cook the creatures from the inside-out. Gray Jacks are the final stage of this process and are near-death, resulting in a necrotic appearance and a perpetual, characteristic odor of decaying flesh.
Despite these Chimera being unarmed and their frail appearance, they should never be underestimated. They possess a long reach and wicked claws, giving them a devastating close-range attack. Intel states that those wounded by a strike from a Gray Jack suffer injuries that are extremely difficult to sterilise and highly prone to infection, resulting in a high mortality rate. Gray Jacks are rarely, if ever, seen on the front line of combat, implying the Chimera do not regard them favorably as military units.
After the events of Resistance: Fall of Man, Gray Jacks are reported to be seen very rarely among Chimeran forces. It is believed that they are "dying" out, and have already begun to vanish from Europe as well as the Menial and other older forms of Chimera. It is not clear why Hybrids no longer grow as they age in later games. A possible explanation is that the Chimera have modified their conversion process, so that Hybrids no longer do so.
Gray Jacks first appear in Cheshire - "Northern Command" where Nathan Hale fight his way through the base and heads for the cell containing the captured Angel. They are initially quarantined in large glass specimen tanks, however they break out of their confinement one at a time, and start attacking the player and a lone surviving soldier, if the player can save him from being strangled to death. They also appear later on in Bracknell, in the underground tunnels where they attack in small groups. By no means should Grey Jacks be taken lightly, in spite of their elderly state. They possess a terrifying ground speed, and due to the reach of their claws, they are also very lethal.
- An effective tactic is to use a Rossmore 236. The Grey Jack will flinch back on the first shot, if close enough, giving time for more shots to bring it down.
- Sapper mines also work well, as the Gray Jacks will always mindlessly charge towards the player.
- A surprisingly effective method of taking down Gray Jacks is to use the standard melee attack. This works for any weapon as the melee attack Gray Jacks absorb can cause them to flinch back, allowing another opportunity for players to kill them. This method should enable the player to take them down without losing a single sliver of health. The best weapon to use would be the Reaper Carbine as they have two melee attacks unlike most weapons (one per carbine), thus enabling a quicker takedown.
- In Cheshire the Fareye's secondary fire allows the Gray Jacks to be easily taken down with headshots as they emerge from their tanks. While it might look like it would work, shooting the tanks with the Auger while they are lowered or before they open does not damage the creature inside.
- The Bullseye is also incredibly lethal, due to its high cyclic rate.
- Grey Jacks are very silent, even when they charge the player. This calls for the player to be more alert in later levels.
- If attacked by numerous enemy archetypes, the player should try to either kill a Grey Jack as fast as possible, or simply run away and wait for a few seconds. The Grey Jack will actually ignore the player and return to its original position should one break off from combat.