You aren't getting it. He's always been a bad tackle. He wasn't moved to guard because he's a competent tackle and doesn't need any more work there. Example Greg Jerman - a journeyman at best - played exclusively tackle last year, in order to increase his versatility, he played guard with the second unit for most of camp. The key point, however, is that now he is again getting reps at tackle and played there exclusively against Cleveland. He has shown versatility.
So what you are saying is that they are so comfortable with Gibson at tackle that he needs zero reps at tackle in an effort to become versatile? Very hard to believe.