If I need to find just one private key somewhere in 256bit space does it have a meaning to use gpu? Or cpu is enough?
At the moment it is impossible to find a 256 bit private key, it takes thousands of the most powerful GPUs and a lot of time.
That's not even close. It takes more than a sextillion of the most powerful GPUs running for 1000 YEARS to find a single 256 bit private key. And that is with Kangaroo/BSGS methods.