When I work on a photo using the GIMP, I typically follow a technique that focuses either on the Grain Merge filter or the Value filter.
Before I decide the values for any filter, I create two layers, copied from the base picture:
And later I might create some masks or shade layers, and I’ll explain those later.
My base image:
First: the desaturated layer (BW) I often use a luminance style desaturation, since that provides more intersting contrast. I might use the Brightness-Contrast filter on that layer afterwards, in order to boost that layers effect. When it comes to using that layer, recently I use it as a Value layer at full opacity. I used to use it as a Grain Merge layer at about 50% opacity, but I tend to find that effect too bold.
Next, I use a color enhance step on the CE layer. This I tend never to use at full opacity, that would be absurd. I generally apply a color enhanced layer at about 20-40% opacity as visually appropriate. This helps me get to the saturation level I pre-visualized for the photo.
Recently, I’ve been using a channel-mixer layer. In this case, i clone the original layer as I would normally, but I choose to Channel Mixer, and often I pull down the blue and green channels to boost the golden or red hues of the scene. You don’t have to do this much, even 10% for each channel can be plenty. And when I do this, I might apply it as a Color or as a Soft Light layer in the mix.
Some times I apply some sharpening, but most of the time I tend not to, as Unsharp Mask can be a very heavy hammer to wield on a photo.
The combination of a desaturated Value layer enhances constrast, the channel mixer and or the color enhancement layer provides the color boost I typically want. Some sharpening is optional. Please try the technique and let me know what kind of results you find!