I think I've seen GPS somewhere else hehe
Goldfish are the same thing as Betta in the sense of misinformation and people telling you very basic minimal care is ok. Pet stores and wiki will tell you it's ok to put Betta in a 0.5g tank, because they live in dirty puddles. Not true, Betta should have 2.5g or more preferably. Pet stores and wiki will tell you goldfish can go in bowls/small tanks because they don't care about your fish. They are not taking care of it, or spending money on it, or upset when it passes.
With that said, personally I would give 2 adult goldfish a 40g tank, you can start with smaller tanks as long as you upgrade when they need it. Goldfish are messy fish, which produce a lot more ammonia/mulm. Regular tank filters that come with kits may not be enough, or just enough for 2 goldfish. To add more fish could be over stocking, which will throw a tank of cycle everytime you turn around. Do your research before you take new fish home please!
Join some goldfish forums and lurk for information, good forums with lots of people. Please don't get fish you are not able to give 100% of what they need.