Jaxilon I don't if this is how he did it but that effect can be done with bevel / emboss . I think Gimp has something similar. You use a low opacity eraser to cut into your top layer. Different brushes cause different effects. Also it helps to set up the bevel/emboss then do the actual erasing / painting so you can see what your getting. In ps you can go back and tweek the settings at will. ! Don't know about GIMP ?