What piece of gear would you tell a first time deployer to make sure to have?

a really good gerber

No underwear. It's one less thing to clean when you sh*t yourself.

Poncho liner. Don’t leave home without it, aka woobie

flip flops

The best thing above all else is the knowledge of the politicians letting the military kick ass and take names. None of this nation building crap when the country is the combat zone. Quit tying their hands behind their backs and unleash the beast. I think it costs more lives if you don't.

I think that the battle-belt is a game-changer that wasn't available to me in Iraq. Being able to transfer weight from your IBA onto your hips helps a lot, especially if you add a set of high-speed-low-drag suspenders that can be worn under your IBA. With a battle-belt you can carry essentials like ammo, water, an IFAC, and a a drop-down thigh holster for your M9 for when wearing of full battle-rattle isn't required on your FOB. It is similar to the old LBE but is much more comfortable, less bulky, and can be worn in conjunction with your IBA to layer-up depending on your needs.

TRX

I served in the 80's. Things were a lot different then compared to today.
I carried a small Transistor radio.I also had batteries for at least a month in the field.
I also carried a Stanley Stainless Steel thermos. If you were in my track and wanted hot black coffee you got it. Disnt matter who.
Another thing I carried was a Flint and Steel with tinder punks. The whole kit was about the size of your wallet.
Is like to thank my Dad for teaching me how to use that piece of gear.

Bag Pipes!

Sunglasses and a “John Wayne”.