I love GVT. It creates excellent growth from what I?ve seen.
I?m a beginner but I?ll jump in anyway with my two cents. First a question: wouldn?t you go more often than 3 days per week? From what I?ve read: two on one off and repeat. It?s three sets of exercise but not necessarily three days. But, depending on your age, 3 days/week may not be a bad idea imho.
But in answer to your question: GVT (at least in me) creastes deep muscle soreness and so you don?t want to do much of anything in those off days. You just want to recuperate. So a little cardio or maybe calves would be okay. But, mostly, I would think your primary goal is to to take it easy. Remember you don?t actually grow in the gym ? you grow the other 98% of your week?