I would like to ask what will be the future of GPU mining after the next fork and change to be Prove to Stake
There will be other altcoins to mine with a GPU, but not Ethereum.
GPU's will be totally useless in Ethereum's mining process after the fork.
PoS happens by a miner putting up a stake, or locking up an amount of their coins, to verify a block of transactions. The cryptographic calculations in PoS are much simpler for computers to solve: you only need to prove you own a certain percentage of all coins available in a given currency. For example, if you somehow owned 2% of all Ether (ETH), you’d be able to mine 2% of all transactions across Ethereum.
It's all about how much you own ( stake. ).
See more about it here as to why Ethereum decides to change it to POS., https://www.ethnews.com/proof-of-work-vs-proof-of-stake-explained
This site displays all the Altcoins which are mineable using GPU's.