Transparent color

[Ofnuts]

There is a reason for layers...

Other techniques:

#1

• Add a layer mask,
  
• Paint the mask with gray where you want the image to be partially transparent.
  
• You can use Levels on the mask to adjust the value of the gray, and therefore the opacity of the result
  

#2

• Copy/paste the area,
  
• Set the floating selection to "Color erase" mode
  
• Adjust the FS opacity (more opaque for the FS means more transparent for the result)
  
• You can make a new layer of the FS if you exepct to need adjustments later
  

#3 (historical)

• Select the area And Edit>Cut
  
• Use Edit>Fade to restore some opacity