The GP-25 is a attachment available for the AK-47 and for the Galil at the cost of 3000. It has slightly longer reload but slightly faster swap-to when compared to the M203 grenade launcher, but is otherwise the same.
Like it's more common M203 counterpart, the attachment cannot be used with the perk Warlord, like all other under-barrel attachments. Its grenades are not replenished by the Scavenger perk and there are no explosive damage increasing perks like Danger Close, Fireworks, or Sonic Boom. It also cannot be used for the first 5 seconds of the start of a round, to prevent players from shooting grenades across the map at the enemy spawn point, which applies for all grenade launchers.