Hmm, if you are referring to the slight blur I'm pretty sure that I did it with the smudge tool. It's been a while since I did this and I was still very much learning my way around the GIMP. I probably have the source files and can verify this but I'm not sure if that's what you mean by "unique glare". The most obvious place to see what I'm talking about is right at the entrance to the cave where the green is very faint over the top.

Now if you mean all the way around the cavern having that raggedy edge sort of look I'm pretty sure that is just 2 or 3 layers of coloring set on overlay with manual smudging to allow parts of each layer to show.

If I were doing this now I would almost certainly use layer masks and just blur the edges of the mask to allow that faint see through look.

A lot of what I do is just manually pushing the paint around since I don't usually get what I want from just running a filter. I do use filters but then I usually go back and touch up the areas I want to be more specific.

If I'm not answering your question please indicate the area you are looking at and I will take a look at the source and happily let you know.