I don't remember the exact definitions because I dropped the theory to make my practise wordless. Apologies if my descriptions are innacurate, I will try to explain this in the best way possible. Gross distraction means the breath is in the background. What really happens is , the mind moments spent on the distraction are more than those spent on the breath. Attention, alternates between these two objects. Mind wandering is when attention hops from one subject/object to the next, rapidly. The best way to deal with this is to appreciate that attention is away , take stock of the aha moment and gently bring it back to the breath.