Don’t understand your question.
You can make header in any graphic program you have, gimp, photoshop or find and crop photo or any jpg from the web, doesent matter.
Or maybe you are refering to using gimp native file type? If so it will be more complicated i think.
But if WP is accepting (after some modification in the code) file types from Inkscape so i think gimp will also be possible.
I’m asking if I should create a complete header (including logo) in a graphic program (like gimp) and then bring it into GP/WordPress versus trying to create the header (except logo) completely in GP/WP (if that is possible)?
I think I will try it in Gimp… Then how do you bring it into GP?