Gatekeeping isn't inherently misogynistic or conformist, but the motivations in cases like this probably are.

I also don't think it's unreasonable to ask people not to publicly associate themselves with a thing if they know nothing at all about that thing.

Nobody owes anyone an explanation of how entitled they are to wear a band's t-shirt.

I've listened to enough iron maiden to know that I like the band, but could only name one song off the top of my head. Do I have your permission to wear their t-shirt?

You're channeling the same energy as someone that feels entitled to know about a stranger's genitals if their bone structure seems misaligned with the gender they're presenting.

It's so easy to not be an asshole watch this:

Slayer fan:

Iron maiden! Nice! I saw them play at XXXX back in XXXX. It was a wicked show.

Iron maiden fan:

Oh wow, that sounds so fun! I've just learned about them, but I'd love to see them in concert someday. The metal community is so welcoming!

Or:

Oh wow. I don't really listen to them, but they were my cousin's favorite band, she died recently, so I wear this shirt when I'm missing her.

Or:

Oh wow, I was at that show too!

Or:

my uncle is the drummer and is always giving me these

Or:

Uh huh? I just like the colours of the shirt.

Life is so much better if you don't make nasty assumptions.