There is no law reference for the gate, it's not mentioned in the laws (but then you already knew that).
As we have seen from this thread the entry point at a tackle is vague in the laws.
To make sense of it as referees we are told (you will have been told I am sure) that the easiest way to do this is visualise two gate posts.
Take a line across the back foot, then on that line note a point at the left and right edges of the tackle area. If you place an imaginary gate post at those two points on the back foot line, anyone entering the tackle in front of the gate post is illegal. Anyone entering between the gate posts is fine.
It's a way of helping you see in your minds eye what is acceptable. We do it for lots of things in the game in different ways. It's a referee aid.
But you see I know that you are already aware of all this, so why are you trying to muddy the waters...except maybe to prove a personal point?
At this point i'm out and wont be responding anymore.