This instructable will help you to make images seamless in only the dimension of your choice.
Step 1: Open the Image in GIMP .
Step 2: Select and Copy the Left 1/3rd of the "base" Layer.
Step 3: Paste It in a New Transparent Layer
Make sure you paste at the right 1/3rd of the image.
Step 4: Select and Copy the Right 1/3rd of the "base" Layer.
Step 5: Paste It in a New Transparent Layer
Make sure you paste it at the left 1/3rd of the image.
Step 6: Add White Layer Masks to Both the New Layers.
Step 7: Blending the Masks. Step1
(make sure the 2 colors are black and white)
Step 8: Blending the Masks. Step2
Step 9: Blending the Masks. Step3
Step 10: Crop Away 1/6th of the Image From Left and Right Each
Step 11: Testing & Finish
To test, use the "tile" plug-in. It is at various places in different versions. So i wont instruct on how to use it.
These instructions were for making an image seamless horizontally. You can adapt them to make images seamless vertically only too.