Start with an image that has a noticeable background gradient, or a lot of light pollution......or both.

Rename the opened image (so that the original is still on the computer). {File - Save As}

Duplicate the image. {Image - Duplicate}

Method 1 - when there is a brightness gradient with or without light pollution

NOTE: This method can be difficult to use when the object is bright and prominent - M42, for example.

Use {Filter - Noise - Dust&Scratches} on the duplicate image.
Leave threshold at zero, move the radius slider to the right until all detail disappears, all the way to 100 pixels if you want.

Now you're left with a copy of the background gradient/light pollution.