The above 2 methods will help you blur a specific part of the image in GIMP. If you want to blur the entire image, you can use the Blur filters. GIMP offers more than one blur filters (such as ...

So you can apply any filter, say a Gaussian Blur, to all the layers beneath it in one swoop, and control it non-destructively. This, my friend, is tremendous power. GIMP is still playing catch-up ...