Sadly I'm afraid that 2 gallons is too small.
In my head that's around 7 litres, which isn't big enough for any fish, no matter how big or small they are.
A 75 gallon tank would only be good for so long, comet goldfish can grow up to 1 foot in length (often more because of their long flowing tails), they are pond fish and belong in ponds. I would not recommend buying them for tanks.
Goldfish, like most animals, do most of their growing as juveniles, so you want to give them the most space in the first few years of their life so they actually have space to grow properly. Consider it this way, you wouldn't buy a guinea pig and then put it in a hamsters cage, because you know it's going to grow to a considerable size and need space to exercise.
What will happen if you buy a goldfish and keep it in a tank that is too small; it will become stunted, potentially shortening it's life span and in some cases causing health issues. The other issue is that in too small a space the fish will poison itself with it's own waste and again health issues will arise, with a serious possibility of death.
I speak from experience, I had two common/comet goldfish in a 21 litre/5 gallon tank. Within a week one of the goldfish, Star, died.
The goldfish that survived, Lucky, is now living in a 3 foot, 90 litre/23 gallon tank, she is stunted and growing at a very slow rate, when she is big enough I will be upgrading her to a pond, but right now she is too small and will be out competed for food in a pond with bigger fish. I would personally not like to be in this situation, where I will eventually have to fork out a lot of money to keep her or have to give her up.
I am sorry to sound so negative, but I am really glad you have come to ask advice, rather than just buying on a whim
Personally as I already mentioned before, I would use that tank for decoration, with candles or ornaments, perhaps even as a plant only tank.
If you would like to keep goldfish, we would love to be able to give you advice on types of goldfish you can keep in aquariums and what size of tank and filtration type you would need, if you would like to
Once more, thank you for asking for advice, rather than buying without researching first