The trick is to use a good enviroment-map. I created it with Filters -> Render -> Solid Noise (X/Y-Size: 2.8, Detail: 1, Tileable). It is important for a good effect to get different grays in the top right corner. Then I did a Image -> Colors -> Auto-Stretch Contrast and a Filters -> Blur -> Gaussian Blur (IIR) with a radius of 5 to get the full range of gray. Then select the "Golden"-Gradient in Dialogs -> Gradient Editor... and Filters -> Colors -> Gradient Map it to the image.