You will need a version of iTunes that is compatible with iOS 8 to restore your device. Apple pushed an update for iTunes 11 on OSX that supports this today but have yet to push an update for Windows. It'll come alongside the public release next week. The GM is still technically designed for developers.
Edit: Just updated on my iMac running Mavericks with no issues, looking good so far. Can't wait until developers start taking advantage of the new features, that's when this'll really start to shine.