It always depends on the mission, do you mean overnight milsim, more of a one day thing, etc. I can answer generally, and for more specific info find a thread specifically dedicated to it.
Usually you'll wear a bdu uniform of your team/factions colors. So if we take US/NATO as an example you'll likely wear multicam, and carry NATO weapons. 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.
