Your plan already answers your own question in the best possible way:
If you want to be able to make the most possible profit from a large downward move in a stock (in this case, a stock that tracks gold), with a limited, defined risk if there is an upward move, the optimal strategy is to buy a put option.
There are a few Exchange Traded Funds (ETFs) that track the price of gold. think of them as stocks that behave like gold, essentially. Two good examples that have options are GLD and IAU. (When you talk about gold, you'll hear a lot about futures. Forget them, for now. They do the same essential thing for your purposes, but introduce more complexity than you need.)
The way to profit from a downward move without protection against an upward move is by shorting the stock. Shorting stock is like the opposite of buying it. You make the amount of money the stock goes down by, or lose the amount it goes up by. But, since stocks can go up by an infinite amount, your possible loss is unlimited.
If you want to profit on a large downward move without an unlimited loss if you're wrong and it goes up, you need something that makes money as the stock drops, but can only lose so much if it goes up. (If you want to be guaranteed to lose nothing, your best investment option is buying US Treasuries, and you're technically still exposed to the risk that US defaults on its debt, although if you're a US resident, you'll likely have bigger problems than your portfolio in that situation.)
Buying a put option has the exact asymmetrical exposure you want. You pay a limited premium to buy it, and at expiration you essentially make the full amount that the stock has declined below the strike price, less what you paid for the option. That last part is important - because you pay a premium for the option, if it's down just a little, you might still lose some or all of what you paid for it, which is what you give up in exchange for it limiting your maximum loss.
But wait, you might say. When I buy an option, I can lose all of my money, cant I? Yes, you can.
Here's the key to understanding the way options limit risk as compared to the corresponding way to get "normal" exposure through getting long, or in your case, short, the stock:
If you use the number of options that represent the number of shares you would have bought, you will have much, much less total money at risk.
If you spend the same "bag 'o cash" on options as you would have spent on stock, you will have exposure to way more shares, and have the same amount of money at risk as if you bought the stock, but will be much more likely to lose it.
The first way limits the total money at risk for a similar level of exposure; the second way gets you exposure to a much larger amount of the stock for the same money, increasing your risk.
So the best answer to your described need is already in the question: Buy a put. I'd probably look at GLD to buy it on, simply because it's generally a little more liquid than IAU.
And if you're new to options, consider the following:
"Paper trade" first. Either just keep track of fake buys and sells on a spreadsheet, or use one of the many online services where you can track investments - they don't know or care if they're real or not.
Check out www.888options.com. They are an excellent learning resource that isn't trying to sell you anything - their only reason to exist is to promote options education.
If you do put on a trade, don't forget that the most frustrating pitfall with buying options is this: You can be basically right, and still lose some or all of what you invest. This happens two ways, so think about them both before you trade:
If the stock goes in the direction you think, but not enough to make back your premium, you can still lose. So you need to make sure you know how far down the stock has to be to make back your premium. At expiration, it's simple: You need it to be below the strike price by more than what you paid for the option.
With options, timing is everything. If the stock goes down a ton, or even to zero - free gold! - but only after your option expires, you were essentially right, but lose all your money. So, while you don't want to buy an option that's longer than you need, since the premium is higher, if you're not sure if an expiration is long enough out, it isn't - you need the next one.
EDIT to address update:
(I'm not sure "not long enough" was the problem here, but...)
If the question is just how to ensure there is a limited, defined
amount you can lose (even if you
want the possible loss to be much less than you can
potentially make, the put strategy
described already does that - if the
stock you use is at $100, and you buy
a put with a 100 strike for $5, you
can make up to $95. (This occurs if
the stock goes to zero, meaning you
could buy it for nothing, and sell it
for $100, netting $95 after the $5
you paid). But you can only lose $5.
So the put strategy covers you.
If the goal is to have no real risk of loss, there's no way to have any real gain above what's sometimes called the "risk-free-rate". For simplicity's sake, think of that as what you'd get from US treasuries, as mentioned above.
If the goal is to make money whether the stock (or gold) goes either up or down, that's possible, but note that you still have (a fairly high) risk of loss, which occurs if it fails to move either up or down by enough. That strategy, in its most common form, is called a straddle, which basically means you buy a call and a put with the same strike price. Using the same $100 example, you could buy the 100-strike calls for $5, and the 100-strike puts for $5. Now you've spent $10 total, and you make money if the stock is up or down by more than $10 at expiration (over 110, or under 90). But if it's between 90 and 100, you lose money, as one of your options will be worthless, and the other is worth less than the $10 total you paid for them both.