How can I adjust two parts of an image separately without a harsh line between them? [GIMP]

Question

I have a TIFF of the night sky over a mountain (6720x4480), which I stacked using Starry Landscape Stacker. I also saved the stacked sky image "mask" with the mountain cut out, but not PERFECTLY (there's a slight gap between the mountain and the mask that's a little wider in some places than others).

I'd like to adjust the sky and mountain separately, and then put them back together smoothly. I've tried using the mask image to create a selection, shrinking and feathering to try to make the edge smooth, and inverting to get the other part, but no matter what I do once I adjust and reassemble there's either a prominent light or dark line along the border.

I'm completely new to any sort of photo editing software like this, and even the "for beginners" answers and tutorials leave me completely in the dark about what I'm doing wrong. I'm trying to use GIMP (2.10) on a MacBook, partly because I don't want to sink money into software that it's starting to feel like I'm too stupid to use right.

Answer 1

If you want to soften the junction of the two images, you can use a layer mask:

• You add the mask to the top layer, initialize to "Transfer layers alpha channel".
• You will see in the thumbnail preview that the mask is black where your layer is transparent and white where it is opaque.
• Then, painting on the mask changes the opacity of the layer, so the idea is to create a grey edge on the mask make the top layer partially transparent to soften the transition between your two layers.
• The best way is to paint with a soft brush (Hardness 50) and to do brush stokes that get progressively closer to the edge (this is easy to do with a tablet, but still doable with a mouse).
• Physically you don't see the black/white, but the effect they have: whe you paint in black your brush acts as an eraser and when you paint in white it acts as an un-eraser.
• You can tell if you are painting on the layer or the mask by looking at the layer name at the bottom of the window newt to the zoom indicator (which says {layer name} mask instead of just {layer name} when your actions target the mask.
• Right after adding the mask the mask is automatically made the target, but you can switch between layer an mask by clicking their preview in the Layers list, or by toggling Layer ➤ Mask ➤ Edit layer mask