More contrast - let the highlights blitz, no dead (=perfectly flat) piece of metal, nearly all details as own embossed layers for control. The flat areas are brushed that gives to them a complex highlight-shade variation. Without it it's only plastic.
Before you start, you need a plausible reference. Keep it onscreen. It's awfully easy to get lost without it.
Turning the texts and sculpted patterns to gold is well documented, but the plain metal is somehow much more difficult, as you have seen. GIMP's preset "Golden Gradient" has become a cliche' that makes it miserable. Some creative geometric mangling makes it nearly acceptable, but generally it's still too plastic.
Some virtual alchemy helps. A piece of grey metal can be processed to gold in the computer easily - assuming you have a plausible image of it. You need only to add the yellow color. That needs a yellow layer over the grey metal layer. Use a transparent blending mode. Blending mode "Color" is good, because it does not affect the brightness, if everything is kept around luminosity = 128.
The following example has a 50% grey shape and it's copy. The copy is filtered by
Script-Fu > Effect > Chrome Image (it's in the extension pack):
A flat yellow midshade color was copied from the (fake) reference and added to the topmost layer (bucket flood) to colorize the grey shapes; the blending mode = color. If your color happens to be highly saturated, then "Overlay" is a better blending mode.
Sript-Fu Chrome Image takes the chrome reflections from the neighbourhood. This effect can make shiny edges, but it can't create a natural flat metallic surface from emptiness. It needs some texture as basis.
A photo of real metal can be useful. A grey one is ok. Actually it should be desaturated to have the control over the color. Here's a snippet of a photo taken of a not so well treated aluminium plate:
Really sandpapered. A little less grainy polish would be better. As colored it's not especially interesting, altough usable:
The metal plate can be enhanced by filtering. This makes it more usable as the basic surface or a texture for patterns. The following image consists 6 different filterings. The filterings often affects to general brightness, which is compensated by adjusting levels.
1 = Gaussian Blur (quite same as the usual gradient)
2 = Emboss
3 = Noise
4 = Chrome Image
5 = Softglow (=blurred highlights)
6 = no filtering, but taken the best piece of the plate
The numbers are 50% grey, added Bevel and adjusted levels for balancing highlights and shadows.
Here's the final test coin. It's a combination of methods
- the aluminium plate is blurred and mixed with a thin unblurred version. A disc is cutted and beveled. NOTE: a large piece is used to keep the scratches small.
- the text has bevel& emboss. It's blending mode = Hard light to preserve the metal surface texture in midtones Originally the text was 50% grey.
- the levels are stretched to the maximum in the final image
- the fake gold medal was kept as the reference and the color is taken from it