Make a pattern by using a square canvas of 6 by 6 pixels. use pencil tool to draw a diagonal line, save as pattern (should be .pat extension for gimp). Put the pattern file in the folder where patterns are stored. Then you can pattern-fill it using bucket tool.
There is a temporary approach where you can just copy the pattern into your clipboard, then pattern fill, with "clipboard" as the pattern of your choice. For this you wont need to save a pattern but you'll need to redraw the diagonal line everytime