I have lots of gmic files in the root of my home folder (and I don't like to have them there):
There are .updatexxx.gmic files (different numbers for xxx), .compatibilty.gmic, .garagecoder.gmic, .gmic_def.xxx (different numbers), .gmic_faves (guess I should keep that), .gmic_sources.cimgz, .iain_fergusson.gmic, etcpp...

Are they still needed or are they obsolete? And if still current: IMHO it would be much much better to have an own folder for that instead of muddling up the home folder... Is there a way to configure the folder?

Interesting question. I looked in my home folder and found some gmic files too. I'm thinking it would probably be ok to delete them. If they were generated by the gmic plug-in, they'd be generated again if they were deleted.

You can delete all of them, except the faves file.
These are update files, used by previous versions of gmic.

To be more precise about the meaning of each file :

- All files named '.gmic_def.xxx' are old update files for version 'xxxx' of the G'MIC plug-in. They do not exist anymore in recent versions of G'MIC.
- All files names '.updatexxxx.gmic' are update files for version 'xxxx' of the G'MIC plug-in (more recent version). The difference with the previous is that it also includes all filters from external sources.
- All other files (except .gmic_faves) are update files provided by external sources, for older versions of the G'MIC plug-in. They do not exist anymore, since they are merged in the unique '.updatexxxx.gmic' file now.

The '.updatexxxx.gmic' file is the one downloaded by the plug-in, when you click the 'Refresh' button, under the filters tree, so you can recreate it at any time easily. The only file that contains 'personal' data hard to recreate is the 'gmic_faves' file, containing all your Faves filters.
This is probably the only one you need to keep.

Thanks for your answers . Is there a (reasonably good ) reason why the updatexxx.gmic files are kept and not get deleted on a new update?

Each updatexxxx.gmic is associated to an installation of the plug-in, version 'xxxx'.
If you have several installations of the same plug-in (with different versions), then I don't see why the update files should be automatically removed.
This removal should be more handled by the package manager (or the uninstaller on Windows), than by G'MIC itself.

I don't use the package from my distribution because that's outdated, I use the download from the website. So the package manager cannot handle it because it doesn't know it. But now that I know it I can handle it .