I know this is easy to suggest when I have the tools, and they are quite expensive.... Substance Painter.
If you're starting to do serious texture painting, you should drop GIMP. You won't keep your sanity intact with 2D painting.
There are open source or free alternatives you could try. I will give what I remember from the top of my mind, it might be obsolete or outdated, but it will give you a starting point:
- Armor paint
- Quixel Mixer
- Blender
- InstaMat
I'll let you dig and find by yourself, there might be more opensource/free software : )