NYC Airsoft

I'm new here, what kind of gear do people usually take to milsims?

It always depends on the mission. I can answer generally, and for more specific info find a thread dedicated to it.
Usually you'll wear a bdu uniform of your team/factions colors. So if we take NATO as an example you'll likely wear something like multicam, and carry an M4. You'll want to have a backpack with hydration in it, be that a hydration bladder, or extra water, you NEED to have water. Then of course you need your primary gun, secondary. Possibly backups for both. Ammo, mags, propellant (batteries, gas, Co2, HPA tank, whatever your gun needs), you'll need cleaning tools in case anything breaks. A change of socks and underwear because you'll be glad you have it. And camping supplies if you are going for a overnight milsim.

you put on your body whatever you want to as long as you put it on.

How important is it that I replicate the primary gun perfectly? Does my M4/AK have to look like what's in service or is it a general thing? I'm hoping my VFC Avalon is still acceptable for most events for the US/Nato side or whatever https://modernairsoft.com/collections/vfc-avalon-vr16

i would usually go a with a big bad IRONCLAD sort of thing, but cheap version. and would usually have a glock17 and an avalon but it really just depends on the event, like CQB, special mission, single day, that type of stuff