I have just uploaded my first draft:
https://github.com/StefanSalewski/nim-gi
It is the initial milestone -- creates code similar to the c2nim generated ones that compiles on 64 bit Linux with latest Nim 0.16.1.
Of course Jason M. has published a similar attempt more than one year ago already, which even contains some higher level stuff. (https://github.com/jdmansour/nim-smartgi) But I have not much hope that he will continue his effort, and I was never able to compile his code...
Of course it is still a long journey to a reliable high level wrapper, but at least it is a simple working start. (I tried hard to do not too much mud mixing -- so I did only minimal reordering so that Nim compiler does not complain due to unknown symbols. Basically the order is preserved which introspection delivers, that is some sort of alphabetical order with related objects, methods, const and enums located together. Generating the files takes about one minute, I think that is still OK.)
That's fantastic!
Just a single comment though: the gobject world, accessible through introspection, is in no way limited to GTK3. Pretty much all of these are available through introspection:
Said otherwise, your work may turn out to be far far more useful to nim's ecosystem than what you might have expected first. :)