General tip: until you have tons of experience DO NOT USE THE GPU!
GPU processing is a very advanced topic and pretty much a black art currently. The biggest disservice that you can do to yourself is to start using GPU acceleration immediately after installing Reality and Lux. Stay away from it. Use CPU rendering and you will get beautiful images with less effort. GPU-acceleration is an advanced topic that requires a deep understanding of the differences between the CPU and GPU renderers. Build your experience, weeks or months of that, with standard CPU rendering and then revisit the GPU topic once you understand exactly what is involved.