While trying to validate the installation of tensorflow-gpu, I get an ImportError when trying to execute \"import tensorflow as tf\". I am using a Quadro K620 on Windows 7.
After you install the CUDA Drivers you must download cuDNN and release it. After that you get to copy the cudnn64_5.dll to PATH.
If that doesn't work, move the cudnn64_5.dll to
NVIDIA GPU Computing Toolkit\CUDA\v8.0\bin and try it again. Otherwise you'd meet some error that you see.