https://www.gamespot.com/articles/destiny-2-forsakens-gambit-mode-free-trial-this-we/1100-6461881/

Destiny 2's Forsaken expansion has been out for a few weeks and the Raid is live, but that doesn't mean Bungie is done trying to entice people to play the game. This weekend, between September 21-23, anyone with Destiny 2 (which is free this month for PlayStation Plus members) has a chance to try out Gambit mode for free.

In Gambit, two teams of four face off against separate waves of enemies. Fallen enemies drop "motes," which you can collect and deposit into a bank in the center of the map. Once a team has deposited 75 motes, a "primeval" boss appears. The first team to kill its boss wins the round.