removing unwanted portions of selected area in an image

[Ofnuts]

You can combine selections....You make a freehand selection, and either:

• use it in "Intersection" mode around the selections you want to keep.
  
• use it in "subtraction" mode around the selections you want to remove.
  

To set the tool to "subtraction" or "intersection", see the "Mode" row of square icons at the top of the Tool options dialog as explained here.

Usually intersections/keep is best, because you know what you added and you don't need to hunt for the unwanted additions.