when etherum POS starting,GPU miners will cry . 
Yes. But we will find another profitable coin to mine. If the Etheruem changes to pure PoS, its value will not rise as fast.
Miners won't cry because the coin will become unprofitable to mine before it goes POS. ETH has a difficulty bomb to encourage the nodes switch over to new POS software, the network will not approve the switch until there is no more profit in mining.
I give it about a year, by next summer we can expect the switch to happen pending delays and such.
The difficulty bomb will be defused as the PoW mining will continue for some time before the PoS out.
The PoS implementation will be later than expected. So the difficulty bomb will be defused in the futur.e
