Diet will determine mass building. But ive done GVT and its a pretty cool program for building muscle. Once you stop it though and go back to a normal split your strenght will be in the shitter. With gvt you do 10 sets of 10 with some rep timing 2 seconds up 2 second hold at the top then a 4 second negative and then you get like 45 seconds rest. So to do this your going to have to significantly lower you weights and i mean alot. When i was doing it I was repping the 100s on incline for 10-12 and I had to literally drop down to 50s to complete 10 sets of 10 with that rep scheme and short rest period. Then when i went back to a normal split after 8 weeks or so of that I was way weaker, bigger but weaker. took some time to get my strenght back.

Now the 5x5 program is somewhat of a powerlifting program. But its the kinda program where your strength will increase very quickly due to that rep range. But if you eat like a bodybuilder youll make good solid gains. I did something similar when i first started lifting and it was a great program to get my strength up very quickly.

Either way diet will determine the mass building. If you want try 6 weeks of both and see what works better. Whatever you like best use that one for a longer period of time. You've sparked my interest in GVT now. Might have to give it a shot to change things up.