Gambler is an Elite perk introduced in Call of Duty: Ghosts. When spawned, the perk will be replaced by a random perk. The random perk will not be one of the perks the class currently has equipped. It would appear that it does not spawn as a perk set up in a specialist killstreak package.
The perk can provide perks that have no benefit for the class. For instance, a class without a tactical grenade can still get Extra Tactical. However, the perk will never spawn Extra Attachment or Overkill.
Gambler spawns a different perk every time the player respawns, although the same perk can be received multiple times per match from Gambler. Because Gambler's given perk is different with every respawn, Gambler is very inconsistent by nature.
The value in Gambler comes from the fact that Gambler itself only costs one perk point, and is capable of providing perks that would take up a significant amount of perk points. Gambler works well as a slot filler, filling in that last perk point in the player's loadout and potentially giving a huge payoff, such as providing Deadeye, a perk that'd otherwise take up at least half of the player's perk point slots.
Gambler is most consistent on classes that are very perk-heavy which use multiple small/medium perks, as making the class perk-heavy restricts the variety of perks that Gambler can potentially give since more of them are already equipped on the character. It makes Gambler much less random and more likely to provide the same perks that the player could potentially use. On the flipside, Gambler is very inconsistent on classes that have very few perk points and/or perks to work with, as that makes Gambler have a much wider pool of perks to choose from, increasing the chance of giving the player a potentially unwanted perk, especially if the player is using very expensive perks in conjunction with Gambler.
Gambler is one of the perks banned under the eSports mode ruleset in Custom Matches.
Call of Duty Online
Gambler made a return in Call of Duty Online under three variations, one for each perk tier. However, they have been removed.