Is it possible to generate a kind of Van Gogh effect with G'Mic? Like this http://dev.ws.pho.to/de/van_gogh_style/ - with the whirls and strokes... Or is something like that already implemented under another name? The internal Gimp Van Gogh effect doesn't seem to be very close to that...

There's no easy way; been working on this myself, on and off, for well over a decade. I know of no way to do so with GIMP along, but if you add Impressionist (and with a little modification of the pspirc file, it will run inside GIMP), and then a a few G'MIC presets, you can come close. I cannot outline the steps since, w.r.t. these renders, there's way too many nuiances that I refuse to make a tutorial for (sorry). Biggest thing for you is to experiment. Get a copy of Impressionist and, if you don't want to hack pspirc, Impressionist will run inside Irfanview too. Here's an example of where I am currently at. Again, not quite Van Gogh, but it is a one ear profile of my Dad (may he rest in peace).

Here's two others I did not too long ago:

https://www.flickr.com/photos/34520999@N05/13621671963/

https://www.flickr.com/photos/34520999@ ... otostream/

Looks interesting! But I hope there will be another way than installing a Photoshop Plugin... I wonder which algorithm is used by the pho.to effect...

By now I just tried to get a grip on gimpressionist - though I got some nice effects I didn't manage to get the results I was looking for...

Not Gimp, another small java application, (an old one) Painterly

https://github.com/thegrandpoobah/painterly

Interface is this:

Comparison original / painterly