SiSoftware’s Sandra benchmark’s online database has an unknown GPU based on Nvidia GM-200 architecture listed. No actual model was mentioned, but the performance shows an obvious boost of 50% over its current flagship GeForce GTX Titan Black.
The unknown GPU with Nvidia GM-200 uses 3072 stream processors, 24 streaming multi-processors along with 3MB L2 Cache, 12GB GDDR5 memory and 384-bit memory bus. The card was clocked at 1.1GHz base clock/ 1.39 GHz boost.
Who did it? Nobody knows! But the listing was up on 6th November, 2014. Earlier, it was reported that AMD Radeon is working on R9 390X, but that would be coming out by 2015. Most likely, assuming Nvidia plans to release this unknown GPU, would be competing in the near-future as a flagship.
Edit: According to EXPReview, the unknown Nvidia GTX GPU is Titan II GPU.