As far as how confident I am on this guess, I’m going to say I’m at 80%.
With that said, I do think there’s enough compelling evidence here to say Jake Paul is definitely a strong possibility and the voice of Grandpa Monster sounds very similar.
Ultimately Grandpa Monster could also be some professional athlete that’s far less obvious than most other celebrity guesses.
If it does happen to be the Team 10 YouTuber, I’m personally hoping he’ll call out Conor McGregor or some other pro-fighter after losing and then show up to the fight dressed in the Grandpa Monster attire.