There is already one example:
https://stackoverflow.com/a/78990144/22768315
The point is that the string must follow exactly the following scheme:
"[FAMILY-LIST] [STYLE-OPTIONS] [SIZE] [VARIATIONS]"
Further explanations can be found in the documentation of GTK4:
https://docs.gtk.org/Pango/type_func.FontDescription.from_string.html
Have fun testing.