I think a GPU is overkill. In the guitar world, there are dozens of insanely high quality reverb pedals and they all only have some basic chips in them, no GPU necessary.
No, but the question bears discussion. Many of us have high powered GPUs in our machines that sit idle during audio production. Could they be leveraged? Not just for reverb.
My understanding is that latency is usually a much bigger concern than raw computing power when doing audio processing. GPUs are great at doing large batches of computation, but aren't as good at doing lots of small batches with low latency, which is what audio processing tends to be.