How do you add new filters to G'mic?
Pasting the code in the gmic_def.gmic files adds the filters UI to Krita, but when I click apply the filter has no effect.

I have to admit that I don't know....

It is not working right now. So far Krita parses only GUI part of the plug-in and does not register the command part of the filter So it fails when you click apply silently.

So it should work now since this commit:
http://quickgit.kde.org/?p=calligra.git ... 41b650f2b3

Just create new file at ~/.kde/share/apps/krita/gmic/
e.g. myAwesomeFilter.gmic and it should work.

Avoid reserved names gmic_def.gmic * and updateXXXX.gmic ** (XXXX will be different, based on gmic version, e.g. 1590)

* If you have updateXXXX.gmic file around gmic_def.gmic is ignored
** If you edit updateXXXX.gmic, it will be overwritten by next update, you don't want to lose your filters