Or cuda toolkit 7.5 has include the driver already and will take care of installing GPU drivers? According to http://docs.nvidia.com/cuda/cuda-installation-guide-linux/index.html#axzz4KreX3oCK, it seems that I need not install GPU drivers before install cuda. Is it right?
correct, the cuda toolkit includes a driver (352.39 for CUDA 7.5 on linux) for the GPU