What do you need to master to become a profitable player?

Hello everyone, I just got into poker and I have fallen in love with it. While I don’t have any aspirations to play professionally for a living, I would love to have this game as a little side gig where I can make some money.

So with that, I started studying recently a lot of Johnathan Little’s courses, particularly Master The Fundamentals and some GTO. But now after being a part of this community a bit longer, it seems to me that some people actually scourge GTO? And I’ve seen some people say to not even bother learning it. Which confuses me as I always thought knowing GTO was basically a mandatory law to even be close to being profitable. So my question is, is GTO a good thing to know or not, and what is it that profitable players study and mastered that made them profitable?