https://www.gamespot.com/articles/destiny-2-forsaken-update-makes-ace-of-spades-much/1100-6461940/

Bungie has rolled out another patch for Destiny 2 on PS4, Xbox One, and PC. This one, hotfix 2.0.3.1, fixes a handful of issues still lingering in the game following the release of the Forsaken expansion, but the most notable change it makes has to do with how to get your hands on Cayde-6's Exotic hand cannon, the Ace of Spades.

The aforementioned gun is the reward you earn for completing the Cayde's Will quest. You first receive it after clearing the Forsaken campaign, but in order to restore the gun to working condition, you'll need to complete a few more tasks before you're actually able to wield it. One of these involves using a hand cannon in Gambit to kill five invaders--other players who infiltrate your side during the mode.