Well this certainly got interesting.
I thought this was simply going to be a five-minute trek where Grandma makes you do something bad, and with a visual style and overall atmosphere like this, that would have been more than fine. I especially like the piano keys used for dialogue noises, communicating so much just with which key they are playing.
But then, on a third playthrough, it quickly becomes a lot more intriguing, which I really appreciated. I kind of wish you got to see more about some earlier owners of the Toy Box.
My main complaint is your dialogue text. It either needs actual dialogue boxes or a much more readable font - it just takes up too mcuh space, and is simultaneously too hard to read. It also had a tendency to randomly skip itself, which is mildly frustrating.
But man. Good Boy was a much more interesting experience than I expected, kudos to that!