For bitcoin at least, a top of the line ASIC miner can do ~1,000,000x more hashes per second at ~1/100th the power per hash compared to a top of the line GPU. The current total hash rate for the Bitcoin network is ~200EH/s. So a 51% attack would require ~2 trillion GPUs consuming/costing 100x as much power as the Bitction network does today.

I stand corrected Nvidia doesn't have that many GPUs.