If it has a square head on it, often with a
line or arrow on it, and opposite of that square head is a locking nut you have what is called a square head **** or alternatively, a throttle valve. These are made of brass or bronze and can be just a bundle of fun to turn after they've set a while.
First off you need to back off the locking nut on the bottom of it until it is past flush with it's threaded shaft. You are going to beat on that nut and you want to protect the threads from your hammer blows. Start out tapping on it firmly. Then try to turn the square head with a large crescent or open end wrench. You should 'backwrench' the valve while trying to turn it open. Backwrenching merely is the placement of a pipe wrench on the valve, or adjacent plumbing, and creates a force in the direction opposite to which you are trying to turn the valve. In addition to giving you leverage, it prevents your torque from snapping off the thru-hull or something else. The valve itself is heavily constructed and should not break.
What you are turning inside is a tapered plug with a slot for an opening, and the nut when drawn up tight pulls that taper down into the valve body and that is what makes it so tough to re-loosen. Pounding against the nut should be oppossed by a ballpeen hammer on the opposite side of the valve, off the square head, in much the same manner as backwrenching, to limit the transfer of your blows power and confine it to the valve. You can heat it with your torch if it is not full of water. When heating with the torch, whether full of water or not, be mindful of heat transfer to adjacent fittings. Fiberglas and most marine sealants do not take kindly to high heat. It often takes a pretty good rap to get these to move, and sometimes removal of the entire valve, and placing it in a vise is the easiest thing to do.
While apart you can silicone grease the inside of the tapered plug, but it will probably be just as tough the next time. Tighten the nut to the point only that the valve does not leak. Better yet, replace the valve with a ball valve or butterfly valve. These turn freely and, in fact, I recommend removal of their handles to avoid inadvertent closing/opening. The handle can be secured to the hose adjacent to it's valve with a scrap piece of wire.
If you have a gate valve, you may be able to remove the packing nut first, and then the bonnet, to extract the gate and examine and lube it's threads. You will probably want to replace the gate valve, even if you get it to work. Gate valves have gotten a bit of a bum rap of recent. A top quality gate valve has a threaded shaft that actually protrudes out through the valve wheel. This feature keeps the working portion of the shaft out of the fluid the gate is in. It also offers little resistence to flow. Most lesser quality gate valves have the threaded shaft internal to the valve and they set up, break, jump a thread, and impede flow. Other than that they are fine! lol Most gate valves get damaged, in fact most all valves, by over-tightening. You can warp the gate or jump a thread easy enough. They can be quite serviceable if you "exercise" them monthly by opening and closing once or twice. Since nobody ever does that their most oft found position is stuck open, closed, or the always useful closed but leaking profusely. When replacing you will find that a quality gate valve will often exceed the price of a decent ball valve, and be more in the price range of a butterfly valve.
I am not a fan of square head cocks. If for some reason you need to close it in an emergency you will invariably find it set up, and you without the time to be monkeying around with it. They are not cheap either.
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