Fading around irregular image

[Ofnuts]

• Use the freehand select to make a selection that roughly follows the outline of the subject, where you want it to be half transparent in your final image.
  
• Select>Feather and give the amount of fade you need
  

then:

Method #1:

• Edit>Copy, Edit>Paste, Layer>To new layer (Ctrl-C, Ctrl-V, Ctr-Shift-N)
  
• Remove/Hide the original image
  

Method #2:

• Layer>Mask>Add layer mask and initialize to Selection
  
• Layer>Mask>Apply layer mask when happy with result
  

A slighlty more efficient method if you understand layer masks well:

• Use the freehand select to make a selection that roughly follows the outline of the subject, where you want it to be half transparent in your final image.
  
• Layer>Mask>Add layer mask and initialize to Selection
  
• Remove the selection, and  use the gaussian blur tool to blur the mask. In 2.10, the Gaussian blur will give you with a direct feedback so you can very easily adjust the fade.