Using GIMP, what is the easiest way to repeat an image in a tiled fashion?

1. In the menu, navigate: Filters → Map → Tile....
2. Disconnect vertical and horizontal scales (break the "chain" between them).
3. Switch the units to percentage (%).
4. Enter 500% for the width, 100% for the height.

Don't have GIMP at hand right now but have you tried: Filters > Map > Tile?