I don't think it's as much to do with physical developments (either his build or the fewer amounts of slashes, etc.) but mostly mental, although his skating is noticeably more agile and he wasn't a slouch in that department before this season.
We all knew Gaudreau sees the ice very well but last season he pigeonholed himself with repetitive buttonhook plays along the boards upon zone entry, limiting his potential to create offence. Teams noticed his tendencies and locked his options out of reach on a nightly basis.
He's been much better at entering the zone with the puck, then skating to the middle just above the circles before dishing it off or skating further in, and the results obviously speak for themselves. Whether he's developed this methodology by himself or with the assistance of coaching, it's made Gaudreau even more dangerous and defenders give him too much room to work in addition to the newfound areas that he gives himself to work in.