Whatever that something that it actually does in the real, physical world is produces the cogito in cogito, ergo sum and I doubt you can get it just by describing what all the subatomic particles are doing, any more than a computer or pen-and-paper simulated hurricane can knock your house down, no matter how perfectly simulated.
A pen and paper simulation of a brain would also be "a thing happening" as you put it. You have to explain what is the magical ingredient that makes the brain's computations impossible to replicate.
You could connect your brain simulation to an actual body, and you'd be unable to tell the difference with a regular human, unless you crack it open.
I'm not. You might want me to be, but I'm very, very much not.
Of course a GPU involves things happening. No amount of using it to describe a brain operating gets you an operating brain, though. It's not doing what a brain does. It's describing it.
(I think this is actually all somewhat tangential to whether LLMs "can think" or whatever, though—but the "well of course they might think because if we could perfectly describe an operating brain, that would also be thinking" line of argument often comes up, and I think it's about as wrong-headed as a thing can possibly be, a kind of deep "confusing the map for the territory" error; see also comments floating around this thread offhandedly claiming that the brain "is just physics"—like, what? That's the cart leading the horse! No! Dead wrong!)