Edit only alpha layer in GIMP

To edit the alpha channel, add a layer mask and apply the brush effect to the layer mask.

Under the Layers tab, right click the layer to edit and choose Add Layer Mask. A dialog box will ask you how you want the layer mask to be initialized. Presuming you'll start with a visible image and brush away the part you want transparent, the best choice is White (Full Opacity). Click Add.

Under the Layers tab, you should now see the white thumbnail of the layer mask, next to the thumbmail of the layer image. Click on the thumbnail to edit the layer mask. Choose your brush effect and the color black, and apply your brush effect.

If you switch to the Channels tab, you will see in real time how the alpha channel changes as you paint on the layer mask.

If you prefer to start with a transparent image and brush on the part you want visible, just start with a layer mask which is Black (Full Transparency) and apply your brush effect to the layer mask with the color white.

You can always switch back and forth between black and white paint to add or remove transparency on the layer mask.

When you want to return to editing the layer image instead of the layer mask, go back to the Layers tab and click on the thumbnail of the layer image.

Just another method I found using a buggy appimage (eraser and masks not usable):

If you open the channels and deactivate the RGB channels, you can make hard selections on the image and use the fill tool, editing the opacity for gray-values.

Just a simple solution only if you want to change the transparency of the layer. Choose eraser tool. There is an option to change the opacity of the eraser under tool options. Just adjust it to your requirement(say 75). Now apply the eraser.