Gimp can be more powerful than most people think. I found a very useful way to create normal maps with gimp and it’s easy and free also.
If you’re an artist that work with 3D application softwares like blender, 3dsmax, etc, you can take the advantage of this feature to make your 3D art more realistic and the best part is the fact that you use it for free, just install gimp (you can download gimp in the downloads section) and you will need to install a plugin in order to start using the normal map option in gimp, you can download it HERE. If you are a Linux user, you should be able to get the plugin via Software Center.
Here I’m sharing a video about how it works and how it looks.