You should be able to use an image mask to get the effect you want. Masks are non-destructive ways of editing photos. A mask is the same size of a layer it belongs to, but its grayscale only. Anywhere that is white will be fully visible (opaque), black will be invisible, and any grey will be somewhere inbetween. 50% grey will be 50% opacity.
Import your image into gimp,
Right click on the layer, and go down to Add Layer Mask..
Use White (full opacity).
In the layers pane you should see two boxes, one with the image (Red, Green, Blue channels) and the other is the mask/opacity (Alpha channel).
If you click on the left box (the RGB Channel) you will edit the image, and if you click on the right box you will edit the alpha/mask channel.
If you right click on the layer, you can find out if you're editing the layer mask or not. There will be a check next to the "Edit Layer Mask" if you're editing the layer mask.
Paint some grey on the layer mask and you'll be "Painting transparency"! All you have left to do is export it to a format like png that supports transparency.
In the image below is the mask for the image above