With Gimp, I would say export to PDF png for something like this. .jpg is a very lossy format who’s artifacts get picked up by GF.
I’d also recommend getting inkscape (free), as you will need vector lines for cuts and scores when you get to them, and I don’t think Gimp is able to make those, and you can export as .svg which the GF seems to like most in my experience.
Here’s the tutorial page, which goes into far more depth: