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 class without a tactical grenade can still get Extra Tactical. However, the perk will never spawn Extra Attachment or Overkill.