The Retractable Gauntlet-Knife is an assasination weapon used by UNSC agents. If used correctly, it is the perfect weapon of an assasin.
The Gauntlet Knife is actually a whole Gauntlet, with the knife itself inside the gauntlet. When pushing a button on the gauntlet, the knife's sharp edge comes out of the gauntlet, and the wielder can start attack with it. Since it isn't held in the hand, a wielder of the Gauntlet-Knife can use other weapons at the same time if neccessary, which makes it a perfect weapon for agents.
It's the perfect silenced assasination weapon, which is why it's used by assasins and agents.
You can wield other weapons while having the blade retracted out of the Gauntlet, since it's not held in the hand.
It can't be used at long ranges. Because of that, agents will still have to have long range weapons with them.
It is supposed to deliver one-blow kills, but if it doesn't hit a really vital spot, it's not certain it will kill the enemy, since it's very small.
It is a little bit similar to the retractable knife used by Altair in the game Assasin's Creed.