I was watching this tutorial and he suggested G'MIC. There is a filter called debanding noise, I need it badly. I was trying to do it manually but I can see the previews are working. Just the plugin has errors and crashed once. I really need this or a filter from another plugin that works in 2.10.20. I installed it to the default location. Here is a screenshot of the error ( I had a different error with a different filter but this filter is what I needed).
And the before and after screenshot (preview can show "difference").
I am using linux and gmic_gimp_qt (latest) and I do not get that error message. I did a bit of pixel measuring, reckon your screenshot is about 400% After resizing the repair -> banding-denoise filter worked ok, but not really a definitive test
If you have recently upgraded either or both of Gimp / gmic_gimp plugin I would check that you have not inherited any old gmic files from a previous plugin. Some times old versions installed in the Gimp systems plugins folder rather than the User Gimp plugins folder. Unlikely but worth a check.
Everything clean? I have had a good search around and nothing really suitable came up for that type of banding.
Somebody might come up with a good procedure.
For the gmic_gimp_qt plugin error. The author of the plugin answers posts on
https://discuss.pixls.us/ forum. I would ask there.