[eswat]

Most of these GPUs are mining Ethereum and not Bitcoin (ASIC is far more efficient for that). Many of the popular altcoins are not using Proof of Work either, so banding together to 51% attack them using GPUs would be moot. Ethereum will be moving to Proof of Stake later this year (maybe) as well.

[Wassimo]

It would have to be starting with smaller coins with less marketcap that are easier to mine. Since some people would be scared of running a miner the miner used to start the attack would have to be open source and auditable for people to trust running the program.