@marsha Just out of curiosity, is this a standard goldfish like a comet or is it a fancy type?
I agree about taking it inside for a little "vacation." I also understand your concerns about a sudden change of environment. If you don't have a large enough aquarium tank for it, you can use a large tote [depending on the size of the fish, but I wouldn't go any smaller than 20 gal.]. Treat the water with dechlorinator and let it get to room temp. If it's cold, the fish should be easy to catch, which will minimize
THAT particular stressor. Place it in a bag or container [something that will float] filled with pond water and float it in the new water for about 30 minutes before you release it. Be sure you have aeration. The fish is going to be there for a few days, so you'll be doing [at least] daily water changes. Filtration is a good idea, but without a functional biofiltration, it's more for mechanical purposes. Personally, with it being winter, I would keep the fish inside until spring, but that's up to you. Follow everyone else's advice regarding the feeding. I had a newly purchased GF that developed SBD. I noticed it was a "gulper" and assumed it was getting air. Once I started it back on regular feedings, I either gave it sinking food, or would pre-soak it's floating foods [which helps with digestion]. There's a lot of good reading out there about SBD [which isn't actually a "disease," but a condition]. And, as
@Meyer Jordan mentioned, if there is no improvement, you might want to consider euthanizing it.
Sometimes fish just have health "issues." We have to decide what's best for them and if we are willing and able to carry out the terms. I currently have a GF that has become a permanent indoor pet simply because it wasn't thriving in the pond. No real idea what is wrong with it, other than it seems to have visual problems which was making it hard for the fish to compete with the other fish for food. Happy as clam now that he ["Larry"] has his own apartment and a personal valet.
Oh, and BTW, thank you for posting your water parameters!