GIMP — How to set up a file for a commercial printing company.

"GIMP (the GNU Image Manipulation Program), just like Adobe's Photoshop, is really a photo manipulation software program. This means that GIMP is NOT realy designed to create brochure or business card or other printing files. Therefore, you will need to make a couple of adjustments to your GIMP file before you start any of the design process. This page will assist you in making those pre-design adjustments."

http://www.theinternetprinter.com.au/info/How_to_Set_up_GIMP_Files_for_a_Printer.aspx