There are a lot of US gold coins out there that are being sold as collectible. Technically, anything is collectible. The thing that matters is whether or not whatever you're collecting is likely to gain in value over time. Where US gold coins are concerned, there is a good chance that they will increase in value. These are some of the most popular coins in the world among collectors and, thus, they tend to do well where numismatic value is concerned.
There are different types of coins that you can invest in. Bullion coins are coins that get their value chiefly from their bullion. Semi-numismatic coins are somewhat valuable in terms of their collectability and gain a great deal of their value from their bullion. Purely numismatic coins don't have bullion value, but are valuable among collectors. That last category is usually what people are talking about when they say collectable coins. For example, they may be talking about Wheat Pennies, Double-Dye Pennies and other types of coins that aren't very valuable in terms of metal content, but which have a great deal of value outside of that.
Some US gold coins are particularly popular with collectors. The Gold Eagle coin, for instance, is a currently-produced coin that always sells well. There are older coins that sell very well among collectors, of course, but the price of these coins tends to get much higher than their bullion value. At this point, you're not so much investing in gold as you are investing in coin collecting, which is an entirely different type of investment with its own concerns and its own market forces.
If you're interested in buying gold coins, remember that US gold coins are usually very good in terms of performance. There are current-year coins available that usually come at a minimal markup. There are also older coins that are highly collectible. If you're interested in these rarer coins, consider getting them certified so that you can be sure that you're getting the real thing. Certification is done by a third party and involves examining the coin, giving it a grade—which is a measure of its quality—and sealing it in a slab so that it cannot be passed off as a different coin. This is the safest way to buy a collectable coin. New coins generally come with all the paperwork you'll need to verify their authenticity.