Wow! Just wow. How do you guys do this?
Well, what steps did you take with your processing?
The underlying concepts of what is being done are pretty much the same I think, regardless of platform. But they may be done in different ways, with different names, perhaps with different order required, as well as different levels of being "in the weeds" - meaning how much manual vs. automated work is required. Gimp will be fairly similar to PS, for which there are plenty of guides and tutorials out there.
I could post a detailed log of what I did, which could be replicated if you have Startools (or tweaked to your own liking of course), but from a more general overview standpoint, this is what was done with your files:
L, R, G, and B opened and combined, with a synthetic L frame created and added based on the weighting of your integration.
Superpixel binning (scaling) down to 25% of the original size, which improves SNR.
Cropped for stacking artifacts and/or desired FOV.
Gradient and vignetting clean-up.
Contrast enhancement, in which I also compressed the dynamic range of the background (shadows).
High dynamic range (HDR) enhancement.
Deconvolution (recovers for atmospheric distortion).
Color balancing and saturation.
Star shrinking and halo fringe taming.
Combination background suppression, additional star shrinking, and preliminary denoise.
Go back and tame a few more star halo fringes.
Whatever program you use, once you get used to it, all of that stuff will start becoming second nature to you and you will develop a standard routine. Which may vary depending on the particular target.
There aren't as many tutorials, or available plugin tools written, for Gimp, but there's a few out there. And while I like Gimp and still use it for various things, I didn't really enjoy it for astro processing and so got an astro-centric program designed for this sort of stuff.