It's for selecting the GPU you want NVENC to run on.
Do keep in mind that unless your system consists of nothing but multiple Quadro cards, setting that option to anything but 0 will result in NVENC not working (thanks NVidia!).
Does changing the GPU value in "Studio" (Classic doesn't even have this option) still works? Can I have a GPU recording gameplay if I set the value to "1" instead of "0"?1. Classic is dead.
2. This is a post about Studio, in the Studio forum. Please don't try to hi-jack threads.
3. Classic is not only dead, but completely unsupported. Please update to Studio.