SK Hynix's representative has confirmed that most, if not all, NVidia's upcoming gaming architecture cards will use GDDR6 memory.
SK Hynix's version of GDDR6 will operate upwards of 16 Gb/s and ship in up to 16 Gb densities. For speed comparison GTX 1080 Ti has 11 Gb/s memory. Manufacturing costs for GDDR6 will be 20% higher than GDDR5.
SK Hynix is will start mass producing GDDR6 for NVidia in about 3 months. If we're lucky that could mark the start of next gen GeForce production, but if we're unlucky NVidia could also at first use it for one of their professional products.
Source for news: https://www.gamersnexus.net/industry/3271-gddr6-slated-for-next-gen-nvidia-gpus-3-month-mass-production