@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.

@Nachorella @minecraftchest1 I do that a lot in GIMP: right-click a layer, "add layer mask", and it makes a secondary grayscale layer that works like a second alpha channel, that you can directly draw on, apply filters to, etc. A lot of my stuff has solid-color layers with all the work done in those layer masks.