Author of GPU miner contacted me. It is a miner for Nvidia GPUs and he uses seven of them. I offered him reward for releasing the miner.
Awesome!
Unfortunately for me I only have AMD. Well, at least we might still have a chance to fix this problem.
Why would he release the miner when he owns more than half the network hash rate...? Out of sheer good will and honest intention towards the community?
We did end up with GPU miners for Bitcoin.