Krita - PhotoBash 

I also want to know , I somehow understand that it is like imag texture to slap on models to give them realistic look I think

As the word suggests, taking photos and 'kit-bashing' them together into something as shown in the vid. Kit-bashing is an old term for model making using everyday objects {plastic cups, toothpicks, empty toilet rolls, shampoo bottles etc), left over model parts and/or sprig trees. The first Star Wars movie is the most well known example of kitbashing.

You could try it out see how it goes. I don't use gimp. Never liked it.