GIMP Version: 2.8.14
Operating System: Windows
GIMP Experience: New User

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I've captured an example, like this one, this is perfectly what I want.
but in my case it might not be the human, it might be my products

You can use the Free Select Tool, to make a selection around the person/item you wish to not be blurred.
Once the selection is made, copy the selection and past it to a new layer above the original image.
Then blur the original image as you see fit.

You can also use a layer mask.
You would need to duplicate your image and add a layer mask to the top layer.
Then blur the bottom layer and proceed to paint on the layer mask to reveal the blurred layer below.

If I use free select tool it 'probably does not blur the edge very well . maybe the target object is blurred, maybe the background is not blurred

You need to very carefully select the object to remain in focus. Then copy it to a different layer.
Then blur the original layer below the copy of the object.

Or duplicate the image, blur the bottom layer/image and use a layer mask on the top layer/image to paint on, which will reveal the blurred layer below.
The painting on the layer mask, must also be done very carefully as well.

There is a"focus" blur plugin somewhere. You create a layer that you paint black to white to indicate the distance from the lens (roughly) and then you use the map and the plugin to tell the plugin which range of grey is in focus.

However, nothing beats operating a decent camera.