Sorry, for not getting back on it earlier
It looks very good.
I think it's good to have basics to start with. So, using brushes, selections and layers in Gimp, like in the featured tutorials for Photoshop.
From there you can go on doing the weathering etc, showing what tricks work best in Gimp.
Major problem people now have is they see how they can do weathering, but don't know how to do it in Gimp or PSP, since the tools work a bit different or the tools used in the Photoshop tutorials are not available in Gimp or PSP