@Nachorella You can right-click a layer and Apply Layer Mask to bake it into the main layer’s alpha channel (or Merge Visible Layers to combine all layers and their masks).
I think you *can* work with individual R/G/B channels in GIMP, or at least add a Channels tab where they’re visible separately and you can add arbitrary channels; but I don’t have experience drawing on the channels independently like that. But my gut says it may be doable.
@[email protected] The Pinnacle of Software Installation!