Draw on Outline Layer

[rich2005]

Quote:...Let's say I scan lineart and insert it into gimp like the photo above.

If it is a scan, then do you need to remove the background ? All depends on the scan and the lineart.

Add transparency Layer -> Transparency -> Add alpha channel
Use Color -> Color-to-Alpha to remove the background. Often with line art the artwork is faint so after setting the BG colour, juggle the settings to give solid lines.
Then it is as before, careful painting, or a fill, or a gradient as https://i.imgur.com/9Ho2PTk.mp4