Have you tried having both documents open then click on the guitar and keep it clicked and drag it to your other document?
First you have to make the guitar a layer,
Layer> New> layer from background, then click OK.. now you can take that whole guitar and drag it to the next document you want to place it on.
It will create it as a new layer on the other document when you click and drag.
You'll need to do some resizing for it to fit, and probably wipe out the background if there is any around the guitar.. but that should work?
Does that make sense? I tend to explain things in a confusing way.