As far as I know transparency in an indexed image, is on or off as previously stated and not possible to edit from the Gimp indexed colormap. You can get a simulation of semi-transparency using dithering but only when the image is originally RGB mode -> Indexed
This a simplified image with two of the bands with semi-transprency from RGB mode.
The indexed image:
This a simplified image with two of the bands with semi-transprency from RGB mode.
The indexed image: