One of the most useful ways to convert a picture to black and white is the channel mix technique. This allows for the decision on the importance of each primary colour channel (red, green, blue) to the "weight" of the final image, this is, their influence to the final gray shade.
In GIMP, with your colour picture open, you can see the information on each channel by clicking the three-colour icon in the layers tool window: You will see that "weight" of each ink to the image.
You will have to get used to make an analysis such as follows:
- red channel has very high values in red (obvious) and yellow colour (wood sticks) - they appear very bright in the channel. Green and Blue appear very dark in this channel.
- green channel has quite balanced tones over the objects, but there is little difference between green and blue colours. Red match appears a bit too dark.
- Blue channel includes much texturing on the wood, quite interesting. Furthermore, there is a difference between green and blue, indeed. Red is still dark in this channel.
In pictures like this one, it is usual to use information from all three channels, since each one can bring in some information. In this case, I decide to highlight the blue channel to get the texture in the wood, and keep (in a lower priority) red and green channels.
In GIMP, open the channel mixer. In the version I have, it is under the menu "Colours - Components - Channel mixer", but it might be under "Filters - colour" in your version.
Tick on the "Monochrome" box, to get the final black and white output.
Assign weights to each channel as per the above analysis. I set 25% red, 25% green and 50% blue. Sum should be 100%, and you may even put some negative value (result may be weird, though...)
If you want to try some extreme setting, with very high percentages, you may tick the "Maintain luminosity" check box, that will average the percentages input to avoid "burned" or too dark output.
When you click on "Apply", you will obtain your black and white version of the picture...
Setting decision for the channel mixer is your decision, based on your image and your personal preferences. Remember, all settings are valid, and you are the artist...
Once you have your monochrome photography, try to colour it with the "Multiply" option in the layers' fusion tool...