The process is quite simple as long as you have experience of using image editing software. Gimp will be fine but you need to understand how to use the tools to cut out parts of an image and how to work with layers. If you are new to using image editors then spend some time with the basic tutorials that can be found on YouTube. The subjects to look for are background removal, layers and working with the selection tools.
Once you have a good understanding of how to work with layers you will only need to remove the background of your source image and then cut out each body part and save it as a separate layer as shown below.
Then when you have all the body parts ready you can follow the tutorial below if you are not able to use the PSD Templates. This allows you to change body part sprites in CTA3 itself.https://youtu.be/kN-mKg5KlmM
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