by default, GIMP saves in its own format (XFS). But if you just type ".jpg" to the end of the file name in the Save or Save As dialog box then it will prompt for some other detail and save as a JPG. You can also save as a GIF file or a bunch others.
When working with GIMP+Inkscape I usually save the 1st raw file (the Filter/Artistic/Photocopy) as a JPG, then open in Inkscape and "Trace Bitmap", save as SVG (scalable vector graphic), open the SVG in GIMP, finish editing, then save the final as GIF.