I have thought of this but not done it yet, the custom plate required would be simple to make for any machinist, and would probably take about 1.5 hours.
The diameter and thickness of the piece you start with will affect the price, aluminum is around $5 per pound for small quantities at bulk suppliers.
You will also need a custom mounting screw to reach up through your custom plate.
This is all assuming that your Gitzo has the "split-collar" style clamp at the top of the legs that grips either the flat base, or rapid column...you would use that clamp to grip the reduced diameter on the bottom of your custom plate.
You may or may not be able to modify the stock Gitzo flat plate, which will be a quicker, cheaper job if practical. They don't tend to be perfectly flat on top, so, maybe not.
SKGrimes can do it, I can do it, your friendly neighborhood machine shop can do it as long as you can tell them exactly what you want.