GPGPU was a thing well before that! In 2004, it was ready covered in a few chapters of GPU Gems 1, increasing to 18 chapters of the 2005 GPU Gems 2, which including an FFT implementation.

https://developer.nvidia.com/gpugems/gpugems2/part-iv-genera...

I don’t remember it getting much more traction after that: as soon as CUDA was released, end of 2006, it was game over.