There is an alternative solution if you encounter problems in accessing your Notebook. The idea is to launch the Jupyter Notebook with IP 0.0.0.0, without having to set up a static IP first:

jupyter notebook --ip=0.0.0.0 --port=8888 --no-browser

This will generate a token. Do SSH into another terminal, adding the following flags --ssh-flag="-L" --ssh-flag="2222:localhost:8888":

$ gcloud compute ssh sparrow --ssh-flag="-L" --ssh-flag="2222:localhost:8888"

This is how it associates the URL localhost:2222 with the Jupyter Notebook URL localhost:8888. You can then access your Notebook at http://localhost:2222. You also need to enter the token you were given a moment ago.

This alternative was given by the comments on this blog post by Jeff Delaney, Running a Python Jupyter Notebook on Google Cloud Engine: https://jeffdelaney.me/blog/running-jupyter-notebook-google-cloud-platform/.