A brochure is generally a marketing product that is printed on both sides of the paper and then folded, either in half or in thirds. If you are familiar with GIMP, you can use it to make a brochure. Check with the printer to find out the best file format and dimensions to use, and get started.
Make a physical mock-up of your brochure. Take a piece of paper the correct size and fold it the way you want the paper folded for your brochure. Measure the width of the panels and determine the margins you want to use.
Make a new file in GIMP by clicking on "New" in the "File" menu. Set the page dimensions to match the size of your paper or, if you want to have a bleed, make the page one-quarter inch bigger on all sides than the page.
Drag guidelines from the ruler bar to mark the margins and, if you are using a bleed, to mark the edge of the page. Your guides should mark out each column or panel.
Make a copy of the file by selecting "Save as" from the GIMP's "File" menu. Now you have two pages with the guide lines marked out. One page is for each side of the paper.
Add the text and graphics in their appropriate places, adding each as a separate layer. If you have elements that you want to bleed off the edge of the page, these items should go right to the edge of the document, but remember that the guideline that shows the edge of the page is the approximate location where the page will be cut and you need to give a bit of leeway between the edge of the page and any elements you don't want cut off.
Save both files periodically as you work in the XCF format. When you are completely finished, save them again and then use the "Save as" option in the "File" menu to save them in the format required by the printer of the brochure. If you are printing it yourself from your own computer, you don't need the "Save as" step.