Except, Rogues don't get multiple attacks. Sneak Attack is their "thing" instead of multiple attacks.
Some rogue subclasses gain the ability to make more attacks, but none of those are in the players handbook.
So the only place it would matter is for opportunity attacks.
Interestingly enough off hand attack is it's own skill, but happens automatically if you just right-button attacks an enemy.
Here's the thing though, Great Weapon Master is a toggled ability. It shows up where your Reactions are, where you can turn it on or off.