As of Godot version 3.2.3, GDScript does not have a function that can add commas to floats or integers. So, you need to create a helper function that can process the value and return it as a string with commas.
Below is a code snippet of the method that I use to add commas on floats and integers. I decided to share the function that I use on my projects, with the hope that this will help others as well.
Function For Integers
Function For Floats
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