I've been using GIMP for over a decade for a good chunk of my image manipulation needs, from quickly cropping screenshots, creating the occasional image macro, prototyping logos before opening Inkscape and editing pictures to creating whacky abstract stuff[1].
Thanks! In a way it's a mix of generative and manual art, I guess. It starts with gradients or noise. Afterwards it's layers upon layers of color mapping, filters, distortion, more noise, more gradients, etc. - Basically creating artifacts from lossy transforms/mappings/distorts/layer modes and then building those up again into an image. Rinse and repeat.
Some examples of things I made use of:
Colors > Levels. Mostly to get noise to go across the whole range from black to white, not just some in-between. Multiple iterations of reducing the output levels and then stretching those out to the full range again can create interesting artifacts.
Colors > Curves. Same as above, and also final processing. I used a fixed set of gradients (UIGradients) and usually changed them up.
Colors > Map > Gradient Map. Combining the above with gradient mapping, or combining it with noise can result in interesting artifacts as well.
Colors > Desaturate > Color To Gray. Adds grain, texture and contrast around edges. Lots you can do with that, especially by distorting the result.
Filters > Distorts > Kaleidoscope, Filters > Map > Panorama Projection, Filter > Map > Paper Tile. With the exception of the galaxy one, all the ones in the album are somehow the result of those.
I did most of these in 2019 when I was deeply burned out, and clicking around for hours was all I was able to do. It really was kind of therapeutic.
Thank you to the creators and maintainers.
[1] https://imgur.com/a/45mTATy