Gauntlet is a single-player battle against other teams of Heroes. Character health is retained between rounds, and if a character is knocked out, they're unavailable until the Gauntlet is manually reset by the player.
Every three missions, you unlock a Cache. Gauntlet Gems, earned from the Cache, can be spent by tapping Redeem. The Redeem list will be updated from time to time.
The Gauntlet can be reset once per day. Please note, you may increase the amount of times you're able to refresh the gauntlet each day by increasing your VIP level.
Resetting the Gauntlet puts you back at the first battle of the first sector, and your team is restored to full health. Any knocked out heroes are revived.
UPDATE: Starting with the June 2019 Update, Gauntlet will feature seven Sectors, and will reset automatically every day. Gauntlet Tokens are required to access the last two Sectors. Quick Win Tickets can now be used to clear the first five Sectors if you've beaten them before. As manual resets are no longer possible, VIP 10+ users who get extra Gauntlet Resets will instead get multiplied bonuses.