I know that this doesn't answer the question exactly, but for completeness I'd like to add an Anaconda solution. Provided that an Anaconda environment is present on the system, a new Python environment can be created using conda create -n py36 python=3.6 pip. The name py36 can be arbitrarily chosen (could also be e.g. myenv or tensorflow), the desired Python version (in this example 3.6) is specified by python=3.6.
This environment can then be activated using conda activate py36 (or whatever name you assigned in the previous step). Once the environment is active, you can install tensorflow via pip: pip install tensorflow-gpu. To deactivate the current environment and return to the default environment, use conda deactivate. In this way, you don't have to modify PATH variables.
See also this documentation page for more details on the Anaconda environment.