It is an old-ish filter but as with everything, garbage-in -> garbage out, so you probably need to do some pre-processing before diving into gmic. I noticed shadows in the original image come through as painted areas and look strange.
But it also has a mountain of variables. I would just try and see. Get a good result and keep it, poor result throw away
![Wink ;)](./images/smilies/icon_wink.gif)
This in my (k)ubuntu 18.04
![watercolour.jpg](./download/file.php?id=42707&sid=28688b73413092bea597ea2727f0afe6)
- watercolour.jpg (406.55 KiB) Viewed 3480 times
Not over quick, that took about a minute to render.