Gambit is a mode that's part of the upcoming Forsaken DLC expansion for Destiny 2, releasing near the beginning of September. Bungie has detailed how the rewards system for this new mode is going to work.
The way Gambit works is this: you have two teams of players, both working to kill different waves of enemies as quickly as possible. The rewards system for Gambit is a currency called Infamy, and you'll be rewarded with far more Infamy points if you win a match of Gambit, than if you lose.
You then have an Infamy rank, which increases over time as you accumulate more Infamy points. There are six Infamy ranks: Guardian, Brave, Heroic, Fabled, Mythic, and Legend. Each of these ranks has three tiers that you need to progress through (with the exception of Legend, which only has one tier).
Progressing up these Infamy tiers by playing the Gambit mode is going to earn you some exclusive Destiny 2 gear, but Bungie hasn't detailed these new items as of right now. If you want to see what we made of Gambit when we played the mode earlier this year, head over to our Destiny 2 Gambit preview.
Alternatively, if you need anything else on the Forsaken expansion set to bring in a ton of new weapon and armor items, you can check out our Destiny 2 Forsaken DLC guide. The Forsaken expansion will be launching for Destiny 2 on September 4, and Gambit will be included in the DLC.