It is an attachment featured on the AK-47 Grenadier used by the Loyalists, OpFor and the Ultranationalists during the Singleplayer Campaign. Sergeant Kamarov is featured using this weapon. There is an AK-74u in "F.N.G." that can be seen with a GP-25, however, it is completely unusable.
This is an attachment for the AK-47 assault rifle, which is available from the start, although rank 4 is required to make a custom class that uses the weapon. As previously mentioned, it is indistinguishable from the M203 attachment in function and damage. However, it has a significantly faster switch time than the M203, in exchange for a slightly longer reload time; however this is easily negated when used with Sleight of Hand.
The majority of servers and players look down upon the use of grenade launchers as they are extremely effective and require minimal finesse to attain kills. It has earned the nickname "noob tube" and is referred to as such almost exclusively, even by players who use the attachment. Like the M203, it can be used to attain the challenge "Ouch" by directly hitting and killing an enemy player within the minimum range, which does not trigger the explosive fragmentation, but simply impacts the target, causing an instant death.
In Modern Warfare Remastered, the GP-25's reloading animation has been altered: it is no longer flicked to eject a spent casing.
The GP-25 returns in Call of Duty: Modern Warfare 2. Like in Call of Duty 4: Modern Warfare, it is still only usable on the AK-47. It is similar to the M203 in damage and effect. Unlike the M203 in which the grenade compartment slides forward to accept/release a grenade shell, the GP-25 simply has a hole in front, which the user snaps his wrist to release a spent grenade, and drops in a fresh one, called muzzle loading.
The GP-25 is only available for the AK-47. It has slightly longer reload but slightly faster swap-to when compared to the M203 grenade launcher, but is otherwise the same. When equipped, it gives the AK-47 leaf sights, although they are unusable.
AK-47 Grenadier in Call of Duty: Modern Warfare 2.
The GP-25 is an attachment available for the AK47 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.
This 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.
The GP-25 was first seen in the Survival trailer mounted on an AK-47. Like in the previous games it is used on the AK-47 as its own unique Grenade Launcher. It is exactly the same as the other grenade launchers in terms of damage and range, with a faster switch time in exchange for a slightly slower reload.
The GP-25 is available for the AK-47 upon reaching level 8, along with the other assault rifles. Scavenger does not resupply grenades, however, running over a dropped weapon that has the GP-25 equipped will give the player 1 grenade.
When the GP-25 is attached, a small part of the grenade launcher is visible to the right of the iron sights. This is the quadrant sight for accurately aiming the grenade. Also, the AK-47's maximum reserve ammo in campaign increases from 300 to 308.