Canon_500D - Forums have to do their regular maint, and as this one is worldwide and 24/7, there's no "2am periods" for doing that "overnight"....
If masking is a bit confusing - you can isolate an unblurred area - or more than one - with layer overlay and an Alpha channel - somewhere for it to put the 'change' information until you flatten the image.
This quicky-method is done entirely with 1 JPEG from a camera.
Create your "blurred" version of the image "Filename.jpg" as say, "Filename1.jpg".
Open Filename in Gimp, and as that's "Background", Copy it to a new Layer, Layer-1. Create a new Layer, Layer-2. Import Filename1, the blurred version, to Layer-2. Right-Click the selected Layer-2 in the Layers Dialog, and "Add Alpha Channel".
Enlarge the image in the canvas window to make it easier to work on the areas you need to. You can move the image around in the window with the sliders as you work. Use the Eraser, etc, and reduce the size with the tool's slider in the dialog box on the left. You can greatly enlarge the image - and reduce the brush-size - to do the finer fiddly-bits.
You can reduce the opacity of the blurred layer if it's severely blurred, to better see the outline of the sharp image below it, with the Opacity Slider in the Layers dialog.
When the selecting is complete and checked - flatten the image and save to a new Filename2.jpg. You can save the worked-on image with the layers by first saving it to XCF - so you can open it to modify later if needed.
Not sure if I should put images here - so will just link an example - you'd take the time to be neater - but that gives an idea of what the above quicky-method does.
If that's not actually what's intended - you might explain a bit further.