In the Gimp, you should be able to simply use the elliptical selection tool (E) to select the area you want, and then either copy and paste into another image, or Select->Invert (Ctrl+I) and then Edit ...
The GIMP image editor is not nearly as challenging as you might think. There are several things you can do to improve your workflow. All of these tips are free to use. I've been using the GIMP image ...
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