So here we have the first real gameplay of Gears of War Judgement.
It features footage from the trailer shown at E3, the extended version (as found on this site) and of course brand new footage.
Essentially it looks like Overrun mode is a mixture between both Horde and Beast mode. Those playing as the Locust must attack the fortifications that the Gears are protecting. The Gears however play a more defensive role in this match type.
We also know that the Gears will know be in classes for this match type (soldier, medic etc) and that the Locust will follow a similar style of currency found in Beast mode.
However it isn’t clear if Overrun will replace both Horde and Beast but obviously more information will be released before the game ships in 2013.