Both fancy goldfish and regular goldfish need big tanks, fancy goldfish can get just as big (a few inches shorter due to shape but overall mass wise is often the same)IME, so boboboy is right IMO.
As mentioned though, the answers no unless you have a similar sized tank to overwinter (which would kinda defeat the purpose of using the pond) or can afford the electricity to heat it to above freezing over winter (in which case there are loads of better-suited and more interesting species anyway).
Also, bare in mind that if you were to treat it as a 50gal tank, you would need the equivalent od a big external filter and generally still need to do water changes anyway, fairly impractical for an outdoor "pond" like this.
If you wanted goldfish, you would be better of getting an indoor tank.
This thing is more suited to minnows and the like, set up as a natural tub pond, with loads of plants. Look into legally acquiring small native minnows/sticklebacks, or heat it over winter.