Selecting areas with the Fuzzy Select tool is not a fine art. You will have been left with some white edges between areas of texture.
In GIMP, select the Cobbles layer in the Layer pallet. Right-click and select Mask to Selection.
Go to Select ¦ Grow and input 1 into the box, and then click on OK.
Make sure your Layer Mask is selected. Go to the Bucket Fill tool.
Select the Fill Whole Selection button as shown in the following screenshot, and click in the selected area.
You expanded your selection area by a single pixel and then modified the layer mask to include the new selection area. This meant that any white lines at the edges of the cobbled areas were removed. You can repeat this now for any other affected areas.
Your layer stack should now look a bit like this. The way you have it set up means that you can make changes any time without destroying any other part of the image. Using layers means you're completely safe to experiment...