From the last changeleog :

• Feature – Gauntlet Wars
  • Players can conduct Gauntlet Wars against one another. This also applies to alliances.
  • Gauntlet Wars must be declared and accepted like a contract in order to take effect.
  • Towns of players who you are involved in a Gauntlet War against will be highlighted on the island view with a red-coloured banderole.
  • Islands with player towns against whom you are conducting a Gauntlet War will be marked with a red flag on the world map.
  • In the 'Gauntlet Wars' tab in the military advisor, you can see who you are involved in a war against or which alliances your own alliance has a war against. Each Gauntlet War is linked to its own page which lists the towns of all enemy players. In the case of Alliance Gauntlet Wars, the general can mark priority targets on this screen which will highlight them with a crosshair.
  • During a Gauntlet War, the number of General Points destroyed by each party is counted and displayed in the overview. Damage done to third parties (e.g. in battles where more than two parties take part) will not be counted.
  • A Gauntlet War ends when one side concludes the 'Gauntlet War Agreement'.

To clarify just a bit because the instructions given were unclear on how to start a Gauntlet War, and I had to search a bit to find the answers:

You start a Gauntlet War with another player in a similar way to starting a trade treaty or cultural treaty. It's now a dropdown option when you send another player a message. Once they accept, the Gauntlet War begins.

An alliance starts a Gauntlet War through their diplomat. If the diplomat goes to the external page of another alliance, there is a button to click that will offer a Gauntlet War to that alliance's diplomat (similar to establishing a peace treaty between alliances). If the other diplomat accepts, the Gauntlet War begins.

Hope that helps.

From reading it looks like a way to declare wars in game, no more declaring on the forums.

If you want other players or alliances to know about the Gauntlet War, you'd still want to post on the boards. But it's a nice addition that the game will keep track of total damage now.