EDIT:
I have more or less nailed this down now. The following post will tell you what to do to get the poly paint part of ZAppLink V3 working.
Will edit as I discover more.
Hope this helps GIMP fans. 
I got this working!!!
Firstly don’t define the app to use as GIMP, set the windows default for this file type to open in GIMP.
Other than that you may need to take any non-numberic characters out of the file path, I think the ‘-’ character may cause an issue.
In GIMP I edit Layer 1 not the mask, save the file (I have to hit apply mask as GIMP tells me PSD files cannot handle masks). I go back to ZBrush and it tells me there is no change.
Problem comes where for some reason the first time I save all layers are changed to NORMAL and everything other than layer 1 should be HARD LIGHT.
So when I save I close GIMP go back to ZB and it tells me the file is unchanged I press “return to external editor” or whatever it’s called and then change all the layers, expept layer 1, back to hard light, save again go back to ZB and WHAM!!! IT WORKS!!!
Thought I’d share this with those who want to use GIMP.
I do not understand the way you’re doing it. I want to use Zapplink to create a Characterstrip that opens in the Gimp. It does not seems to work.
Not sure, how far are you getting? What part is not working?
Sorry, it does work. Nothing to do but just choose the GIMP exe… I must have been sleepy. 
qu: