I debugged the script and each part of it seemed to worked separately.
Manually installed latest Python 3.7 for Windows, still didn't work.
Tried isolating just the part of the script that launches gibMacOS, ran it, nothing again.
Then I tried to run Python from command and I noticed it did nothing despite Python being installed and added to the Windows PATH command variable.
The fix is very simple but you'd probably never discover it, you have to install the UWP version of Python from the Windows Store. Installing the Win32 version from Python.com does not work on build 1903. You can have both installed simultaneously no problem.
After installing the store version the original script still fired an error so I copied gibMacOS.command and the Scripts folder into another folder and wrote myself a simply script
@Echo off REM Running gibMacOS python "gibMacOS.command" goto :end :end exit
which I saved in the new folder and called Run.bat
and eh voila, run Run.bat and it works.
Hope this helps someone.
BTW I haven't got as far as Install.py yet, I hope it just works OOTB but I guess I will see in around 30 minutes