cannot find instructions for converting .svg to .ggr

[Ofnuts]

You can load the image in Gimp, then create a path across the gradient, and use one of the several "sample-gradient-along-path" scripts that exists in 2.10. Don't remember if any of these has been converted to Gimp3.

A more hardcore method is to edit the SVG file (which is a text file....), spot the gradient definition (a bunch of numbers) and create a GGR from that (also a text file with a bunch of numbers).