Yeah. The various breeds of goldfish grow to be anywhere from eight to fourteen inches long. They are also known to live for upwards of 30 years. They're probably one of the worst varieties of fish a beginner keeper can get stuck with. I have one myself and I love him, but I doubt I will ever have a tank suitable for a fully grown goldfish, so I've already made plans to re-home him once he gets unmanageable. For five goldfish... Probably the smallest tank that could be considered healthy for them full grown is 90 gallons (340 liters). I saw one at my local aquarium store wintering in a 55 gallon and he absolutely dominated the thing. They're massive.
On the plus side, that little 6 gallon would make a wonderful betta tank, and any intermediary tanks will allow for other types of fish once the goldfish are gone from them!