Use this simple trick and GIMP's offset function to create a seamless repeat.

You can get a totally seamless repeating pattern with GIMP in just a few 

The Tricks and Steps:

1. Create a new image
2. Pick a color for the background (don't worry, you can change it later)
3. Create a new, transparent layer for drawing on and go wild
4. Duplicate your drawing layer
5. Now here comes the magic: go to Menu, Layer > Transform > Offset (or using hotkeys: Ctrl + Shift + O). You can either enter pixels that you want to move or just hit the x/2, y/2 button and your layer will be moved exactly half its width and length, putting the pattern edges in the middle of the workspace.
6. It's very important that the Option Wrap is activated so that part of the image which is moved off the right side of the image reappears on the left.
7. Save your design as JPG, BMP or PNG and you're done!