GIMP :: Making Rusted Metal Square?

make a rusted metal square in Gimp. I managed to google to make a square selection and fill it with a color. Now I would really like to give it a effect like in this tutorial. I can sort of get it working, but my starting square is not good enough and I do not know much about colors. How do I make a good starting square with some gradient added to it (well I can make one in inkscape and export it, but how do I do it in Gimp), and most importantly, what colors would work well and how do I give the rust a rust-color.