Great first effort ! Thanks for showing.
I also have been messing and learning with GIMP ( sadly I dont have any AltAz of my own yet , only camera on a tripod stuff so not on-topic for this one !)
So, if you will forgive me I have borrowed your pic and played with a light pollution tool in GIMP (well actually it does not seem to be called that in the GIMP sites, but from a previous life in electronics I sussed that it was a useful thing in the frequency/FFT domain, this message will self destruct in 1/2hr ) )
(I dont know what the netiquet is in using other peeps pics so I wil show just a cutdown jpg of your original to show the effect but not steal the intellectuals ! ) ( and it was cloudy/raining outside so it gave me something to do, thanks !)
The light pollution is spread across the frame [ and is (in data speak !) called a low frequency signal] and can be got rid of with a suitable filter [ a high pass filter in other speak ] to leave only the sharpish stuff like the stars and condensed nebulosity
The tool you want is in Filters > Generic > High Pass Filter (if it is not there in your version of gimp then go to the addons repository and look for high-pass.scm and put in your scripts folder/directory)
PS Edit ,, if that is all gobbldygook I'll happily do a blow by blow account how-to get it installed but didnt want to teach eggs if a hint is all you need about the high-pass tool
So, I did this, with a bit of increase in hue>saturation as well :-