I dont use GIMP but in the program I use there is a magic wand selection tool where you say how much tolerance you give it. So if you select a point with grey 128,128,128 then you select 30 tolerance then it will pick from 98,98,98 to 158,158,158.
Another option is to take a copy of the image and use a contrast adjustment so that the range you are looking for is black and the rest is white. The easiest way to do that is with the adjust colours using a 'curves' tool where you can select from and to adjustments using graph curves. Then the area selection is just a black and white image again.