Well it depends what you mean by 'white stuff'. It could be white spot disease (Ich) or it could be a fungal growth. Both will require treatment by medication. First you need to establish which one it is so that you can purchase the correct medication to treat it. Ich is a covering of white spots all over the fishes body, whilst a fungal infection is like a white furry growth. You will need to remove the carbon from your filter as this will just absorb any medication you put into the tank. If the fish has Ich then you will need to treat the whole tank as it can easily spread to other fish. If it is a fungal infection then it might be better to isolate the infected fish and treat it in a hospital tank if you have one. Slowly raising the temperature of your aquarium also helps when you are treating diseases. Goldfish will be quite happy up to a temperature of about 26 - 27 degrees celsius. Frequent water changes and keeping the tank very clean will also help, as will adding aquarium salt to the tank. You will be able to buy medication from most good aquarium shops. I hope this helps

If your other fish aren't eating then they could be infected too, in which case I would suggest treating the whole tank.

Well there are various treatments that you could use. Melafix is a natural anti-bacterial remedy which will help with fungus, and there are many other products that are designed to treat fungus and fin rot. If the medication was for a 10 gallon then could you not just use half of it?

Really your tank is a bit too small for Goldfish so I would advise that you get a bigger tank with good filtration if possible, and this would reduce the risk of your fish getting diseases such as fungus.

I would bet that it's fungus, since you said "stuff" and not "spots". So I'd say that you should buy some meds against fungal infections, and if you're having troubles because the meds are for 10 gallons, you can just half that dose to fit your tank.
And while you're at it, get yourself a 20 gallon tank with good filtration, otherwise you're going to have problems with sick fish forever. Each goldfish needs 10 gallons at least.

It's quite obvious that the fish got sick in the first place because of a crowded tank, which leads to stress (=weakened immune system) and high nitrates. Both of these are really bad for any fish, and would lead to death eventually.

Start treating them right away, and buy yourself a new tank and prepare that in meantime!

make sure when on meds take out the carbon