some hints for GIMP
does not have an auto save...so save often
The layers in the right side panel is how you know what "layer" you are working on, the one highlighted in blue is the layer you are currently editing...you may not "see" that layer in the window, if the "eye" icon is not depressed.
make sure the layer tab is highlighted to see the layer stack.
this comes in handy when doing the lines and rivets and over all shapes of the components.
I myself am new to skinning...but I am not "new" to GIMP so anything you have an issue on, ask for I am sure we can figure it out

I have been using GIMP since I started using computers.
Paul