https://www.gamespot.com/articles/destiny-2s-go-to-gambit-gun-sleeper-simulant-is-in/1100-6461848/

One of the best additions in Destiny 2's Forsaken DLC is Gambit, a new mode that blends PvP and PvE. Two teams of four are separated into their own arenas to fight AI-controlled enemies, but periodically one person can invade the other side, killing enemy players. If you've played Gambit for any length of time, you've undoubtedly seen the weapon of choice for a huge portion of invaders: Sleeper Simulant.