Take it with a grain of salt, but Beyond3D forum member Erinyes claims to have heard from a confidential source that NVIDIA's Turing hasn't taped out yet, but that silicon is expected to be back from TSMC any day now.
The expected launch date if Turing is somewhere towards the end of Q3 2018, so there will be at least one new gaming GPU this year. Based on the latest rumors, Turing will be made on TSMC's 12FFN process and will be Pascal's successor.
Additionally, the post indicates NVIDIA's 7nm Ampere is scheduled for the first half of 2019, but this is a chip for the compute market. This means Ampere is the successor to Volta. The launch of 7nm gaming GPUs is said to be delayed, due to the high cost and initial low availability of 7nm wafers.
Erinyes made some accurate calls in the past, so there may be some truth to this.