>I've gained 6 kg pure muscle mass over the summer
Either he's outright lying: Doubtful

Or he's an elite athlete with no fucking clue about training and his personal trainer also have no fucking clue about the amount of water and fat in human muscles: Very likely

Or he's been given/taken some kind of banned substance: Decently likely

As a newbie you can get 4-6 kg pure muscle mass over an entire YEAR. Given how much Elias Pettersson should've been in a gym over his career he should gain 2-4 kg pure muscle mass over a year. And that would've been if he was just lifting weights and not playing hockey.