I think the three a week full body programs are great and have done them more than not. That said, the two times that I did GVT was after a FB3xW routine. It seems to me that the volume was so high in the one workout doing GVT that it was quite a change and quite a stressor. By doing the 3xW program, your body is adapting to high frequency, when you switch it will go to lower frequency and more volume per session. That hurts! trust me, the soreness can be high.
I guess I'm really only saying that the training stimuli is significantly different and the volume per workout rather high. That is enough for me to keep over training in mind.
As I said earlier though, I think by just making the first GVT workout rather easy by lowering the load and then sneaking up on it over a couple of weeks will do the trick. Ultimately only your body can tell you if it is effective.
I'd give it a shot. If it does result in over training, you'll know fairly fast. At that time you can do a couple of things. Take a week off and allow for super-compensation to occur. (If you haven't done this before it can be pretty cool to see yourself grow from a week off) The other thing would be to focus on low volume and high load for a couple of weeks. This will act as an intensification phase after an accumulation phase. Either way you'll succeed.
After coming to a better understanding of accumulation then intensification cycles I am no longer afraid to over train. I just need to be aware that it is happening and switch to intensification.