There are a few ways of making B&W images in Gimp. The "normal" way is (from my experience anyway) is to either use the Hue and Saturation
(Colors -> Hue and Saturation) tool or the Desaturate
tool (Colors -> Desaturate). While both do a good job of making images B&W the end results do look abit dull, have little contrast and no depth.
So now you're probably wondering which tool "should" I use? The Channel mixer
(Colors -> Components -> Channel mixer).
As you can see the tool doesnt have a lot of knobs and buttons so it's fairly simple to use. One problem though is the preview is quite small, so if you're working on a large image you'll most likely be doing alot trial and error editing.
OK so to show you how it works I'll edit one image from color to B&W. The dog in the picture is a German white shepherd called Jack. The unedited picture looks like this.
The first step is to open the image with the Gimp and select the Channel mixer. To get a B&W image click the Monochrome checkbox. Now start moving the Red, Blue and Green controls back and forth and you'll notice that lower values make the image darker (or underexposed) and higher values make the image brighter (overexposed).
What values to use I got by experimenting. There probably is a scientific explanation on how to do this, but I like to play around experiment with diffrent settings and then see what I get. Every image is diffrent so there isn't a setting that will give good results on all images. On this photo I had to be carefull not to have Red or Green too high since it would make parts of the dogs fur overexposed (too white, loss of detail)
After clicking OK I get this.
As a comparison here's the same image in B&W done with the Desaturate tool
Finally to really make the image 'pop' we need to use the Unsharp mask
(Filters -> Enchance -> Unsharp mask) filter, that sharpens the image and also adds some contrast.
When using the Unsharp mask tool be carefull not to over do it. This is especially important when working with B&W. Try setting a high value on Radius or Amount and you'll see what I mean. For this image I set Radius to 2, Amount to 0.31 and Threshold to 7. As with the Channel mixer - experiment! One trick I do is copy the the background layer and do the Unsharp mask on the new layer with the default values, and then change the opacity of it until I get the desired results.
That's it! Comments or any cries of help are very welcome