Your Flask application should be something like below:

import os

from flask import Flask

app = Flask(__name__)


@app.route("/")
def hello_world():     
    return "Hello World!"


if __name__ == "__main__":
    app.run(debug=True, host="0.0.0.0", port=int(os.environ.get("PORT", 8080)))

See this official documentation for step by step instruction: Deploy a Python service to Cloud Run

There is a plugin called: Cloud Code IDE plugin which makes the test and deployment easy. I am using it for VS code, once the initial setups and permissions are taken care, few clicks, you will be able to run locally, debug and deploy Cloud run services from your local instance.

There is even a simpler approach to to run code in the background in gcp and in every linux terminal: using screen linux

Create a new background terminal window:

screen -S WRITE_A_NAME_OF_YOUR_CHOIC_HERE

now you are in a background window in the terminal. Run your code:

python3 mycode.py

Exit screen with the hotkeys and the job will keep running on the background.

ctrl + A + D

You can close all windows now. If you wanna go back and see what's happening. Log again in your terminal. And tap the following.

screen -ls 

This one will give you the list of the created "windows". Now find yours and tap

screen -r WRITE_NAME_OF_YOUR_WINDOW

And there you have it :D You can find more commands here

So I followed Inigo's advice, plus a little code I found elsewhere (starting with python on google plus a quick lab)

Create the VM as per here SSH in Run this code

sudo apt-get update
sudo apt-get install git -y
sudo apt-get install python3-setuptools python3-dev build-essential python3-venv -y

curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
# wget as suggested in the docs didn't work for me
sudo python3 get-pip.py
python3 --version
pip3 --version

python3 -m venv env
source env/bin/activate

Sprocket on the right to upload the files

pip install -r requirements.txt
python3 filename.py

Thanks for the comments and tips gang. This has got me moving