When you buy a futures contract you are entering into an agreement to buy gold, in the future (usually a 3 month settlement date). this is not an OPTION, but a contract, so each party is taking risk, the seller that the price will rise, the buyer that the price will fall. Unlike an option which you can simply choose not to exercise if the price goes down, with futures you are obligated to follow through. (or sell the contract to someone else, or buy it back)
The price you pay depends on the margin, which is related to how far away the settlement date is, but you can expect around 5% , so the minimum you could get into is 100 troy ounces, at todays price, times 5%. Since we're talking about 100 troy ounces, that means the margin required to buy the smallest sized future contract would be about the same as buying 5 ounces of gold. roughly $9K at current prices.
If you are working through a broker they will generally require you to sell or buy back the contract before the settlement date as they don't want to deal with actually following through on the purchase and having to take delivery of the gold.
How much do you make or lose?
Lets deal with a smaller change in the price, to be a bit more realistic since we are talking typically about a settlement date that is 3 months out. And to make the math easy lets bump the price of gold to $2000/ounce. That means the price of a futures contract is going to be $10K Lets say the price goes up 10%, Well you have basically a 20:1 leverage since you only paid 5%, so you stand to gain $20,000.
Sounds great right? WRONG.. because as good as the upside is, the downside is just as bad. If the price went down 10% you would be down $20000, which means you would not only have to cough up the 10K you committed but you would be expected to 'top up the margin' and throw in ANOTHER $10,000 as well. And if you can't pay that up your broker might close out your position for you.
oh and if the price hasn't changed, you are mostly just out the fees and commissions you paid to buy and sell the contract.
With futures contracts you can lose MORE than your original investment. NOT for the faint of heart or the casual investor. NOT for folks without large reserves who can afford to take big losses if things go against them. I'll close this answer with a quote from the site I'm linking below
The large majority of people who trade futures lose their money.
That's a fact. They lose even when they are right in the medium term,
because futures are fatal to your wealth on an unpredicted and
temporary price blip.
Now consider that, especially the bit about 'price blip' and then look at the current volatility of most markets right now, and I think you can see how futures trading can be as they say 'Fatal to your Wealth' (man, I love that phrase, what a great way of putting it)
This Site has a pretty decent primer on the whole thing. their view is perhaps a bit biased due to the nature of their business, but on the whole their description of how things work is pretty decent.
Investopedia has a more detailed (and perhaps more objective) tutorial on the futures thing. Well worth your time if you think you want to do anything related to the futures market.