I've been lifting weights since I was 12 so I know a lot more than both of you about it. And still train 4 mornings a week even now.
A trained guy like Depay is not going to gain 5kg of muscle in 3 months naturally. Even a complete beginner would struggle to do that. Someone with a decent physique already like Depay will take atleast 8 months, and that would be if he had excellent genetics for it.
Probably half of that 5kg is fat gains. This is why bodybuilders use bulks and cuts, as you can't add muscle without adding fat, it's pretty much impossible