It’s been mentioned before: CUDA SUPPORT needed, please insert in TODO. There has been some work on OpenCL support, see OpenCl · Issue #1678 · Beep6581/RawTherapee · GitHub, but things aren’t progressing steadily.
In contrast, @heckflosse keeps pumping out optimizations for CPU at fairly regular intervals. So you’re already getting the most out of RT without using the GPU.