This. Ethereum already forked once. Ethereum GPU could remain its own currency for all of those miners who invested a shit ton of capital into GPU acquisition.
It could, but would it be worth anything? Miners only mine because they can sell the crypto to others for more than it cost to mine it. At current levels that would require quite a lot of buyers for this new coin with no significant advantages over the main branch.
It'd be worth less, but not entirely worthless. I think ETC is a good example of what happens on a deprecated branch.
If the market value of Ethereum GPU is similar to the market value of ETC to ETH, the impact of Ethereum on the GPU market should shrink by 98%.