The Ghent Workgroup has finally released the GWG 2015 Specifications. They are based on PDF/X-4.
There are seven variants (Magazine Ads, Newspaper Ads, Sheetfed CMYK, Sheetfed Spot, Web CMYK, Web Spot, Newsprint). Beside the CMYK version there is now also a CMYK+RGB version. (These two versions were already introduced by PDFX-ready in April 2015).
In a whitepaper the differences to GWG 1v4 (based on PDF/X-1a) are discussed.