Warhammer Community posted an update detailing the announced release date as well as some confirmation on the Gang War additional supplement.
"What is Gang War? This is Necromunda's first expansion, taking the principles in the Underhive set and combining them with the sprawling 3D battlefields and rich, progression based campaigns that defined the classic editions of the game. In this 64-page book, you'll find everything you need to take your game to the next level, from advanced rules to running your own Turf War campaigns with your gaming group. Gang War will be released simultaneously with Necromunda: Underhive, meaning if you like, you'll be able to play with the advanced rules from day one."