How to create the lines for a KotOR II loadscreen in GIMP:
Set Foreground to white.
Select the "Gradient" tool
Set Mode to Normal.
Set Opacity to 15.0
Set Gradient to White-->Transparent.
Set Offset to 64.4
Set Shape to Bi-linear
Set Repeat to Triangular Wave
Select and drag as with any other gradient, for perhaps 7 px or so.
Kudos to Archon II for figuring this out.