Mining pool or single gpu its irrelevent. As long as honest people can get more computational power than dishonest ones, cryptocurrency is decentralised and 51%-honest.
It's not total centralization, but it's a far cry from the level of decentralization that would be possible if mining pools did not exist.
There is a lot about this topic in the new, free book from Princeton authors titled "Bitcoin and Cryptocurrency Technologies" (a great read)[1]. There is much research being done on the front of finding the best way to eliminate mining pools or at least stop them from potentially centralizing consensus to just a handful of people.
[1]: https://freedom-to-tinker.com/blog/randomwalker/the-princeto...