So what does the GamePad add to Assassin's Creed III?
Assassin's Creed III is the first entry in the series to hit Nintendo's home consoles, and at E3 2012, we got to look but not touch a running demo of the game on Wii U.
For those new to the series, the overarching story involves the ongoing fight between the Assassins and the Knights Templar. The first game takes place in the Holy Land (Jerusalem and surrounding areas) in 1191. It also takes place in 2012, as players control Desmond Miles, who is the descendant of the hero of the first game, Altair. Desmond is helping out the modern-day Assassins to recover Pieces of Eden, which can be used to control people's minds.
The second game takes place in the late 15th century in Italy. Players take control of another ancestor of Desmond, Ezio Auditore de Firenze. The story continued through Assassin's Creed II, Brotherhood, and Revelations.

In Assassin's Creed III, which is set to come out on Wii U around the system's launch, players once again play as Desmond, but this time the ancestor he plays as is named Connor, a half-British half-Native American. The game spans a 30-year period before, during, and after the American Revolution across several North American states, telling the story of how Connor became an Assassin.
The game is running on a new engine, so it looks gorgeous. The way Connor interacts with the environment looks better than any prior entry in the series, and instead of things looking obviously like footholds and climbable walls, everything looks more organic.
The Wii U GamePad is currently being used as a large map and an inventory management menu. Normally, there is a small mini-map on screen, but that map is now elaborated on the GamePad. Additionally, your weapon and tool wheels now exist on the touch screen as well. There is even an option to disable the HUD, so you can just do everything on the GamePad and have the TV screen look unfettered.
From there, thanks to the familiar controller design of the GamePad, it controls just like any prior entry in the series. As a matter of fact, you can eschew the GamePad altogether and just use the Pro Controller.

We'll likely find out a whole lot more about Assassin's Creed III in the coming months, ahead of its October 2012 release on other systems. Whether you've played every game in the series or skipped them all, this American Revolution-inspired game is still likely worth paying attention to.