After downgrading to unity 5.10.0-0ubuntu6, I don't have this problem.
With 5.12-0ubuntu1, I had the problem as described - after closing and restarting GIMP once, it no longer appeared in the launcher or in ALT+TAB. Switching to it required closing/minimising all windows in front of it.
After downgrading to unity 5.10.0-0ubuntu6, I don't have this problem.
With 5.12-0ubuntu1, I had the problem as described - after closing and restarting GIMP once, it no longer appeared in the launcher or in ALT+TAB. Switching to it required closing/minimising all windows in front of it.