In music, the pitch-and-harmony-related content matters---that stuff we can capture in traditional sheet music---but it isn't everything, by a long shot.

You might think some pop song is hogwash, until you hear cover after cover that cannot nail it.

There is plenty of music which is entirely incomprehensible to the uneducated listener. This isn’t a good example.

The point is it isn't exclusively that. Nor is that kind of gatekeeping an essential part of music, in fact it is not admired in any way.

But I wouldn’t describe it as gatekeeping at all. It’s just complexity that isn’t immediately obvious to the layman.

Much of contemporary art is the same way. Outsiders without any knowledge may think it’s a giant scam, but there are plenty of logical reasons, historical or ideological or otherwise, for artists making the things they do and collectors buying those things. You might not like those reasons and they might not be good ones, but I’m not seeing anyone even elucidate what they are in the first place.

I think it was explained pretty well in other comments: it's gatekeeping in the name of "protecting the business" where the main business is only one thing: money laundering. Sure, there are people still buying art to display on their walls, but that's not where the big money is. There's exactly zero painters selling millions of copies of their work the way any halfway decent singer would sell, although everybody decorates their walls with something.