On the Xbox One subreddit, Project Spark's community manager Mike Lescault cleared up some confusion when he confirmed that "You do not need an Xbox Live Gold account to play Project Spark." That's not just on Windows 8, but any platform. The game is free. No subscription needed. Simple. Easy peasy lemon squeezy.
Project Spark was announced for Xbox 360, Xbox One and Windows 8 during Microsoft's E3 2013 press conference. Players will use a combination of Xbox One's Kinect voice capabilities and SmartGlass to manipulate the world and input commands.
Registration for the game's beta is open on the website.