Brighten Portion of Image?

[Ofnuts]

Because when there is no alpha channel, erasing is like painting with white?

What you really want to do is

1. add an alpha channel to the top layer:Layer>Tranparecency>Add alpha channel (so this layer can be transparent)
   
2. to be able to tweak the opacity, add a "Layer mask"; Layer>Mask>Add layer mask. In the layers list the mask preview appears near the layer preview. you can select if your tools apply to the mayer or to the mask by clicking on the adequate preview. The status bar (bottom of image) tells you if you edit the layer (say "layer_name") or the mask ("layer_name mask")
   
3. now you can "paint" the opacity on the mask, what is black is transparent, what is white is opaque. In your case you'll use a soft brush so that it produces grays on its edges. You can correct errors by painting in the opposite color...
   
4. when happy with the result: Layer>Mask>Apply layer mask.