It's common for lfs
employees to say fish will do fine in a tank far smaller than they really need. I would do your own research on the matter.
You really should know the size of your tank, still have the box? If not, measure the length, width, height, and post the information on here. We can figure out its size with those measurements. If it's a hex just measure out each section.
You could also take a picture and we could estimate its size.
And yeah, I would certainly change half the water if you haven't already. Gold fish have a huge bio load, and ammonia will rise quickly in an uncycled tank. Make sure you use a dechlorinator when you do the water change.
Until the tank is cycled you will need to do partial water changes daily most likely. It helps to have a test kit so you know how much to change.
I've never kept gold fish, but my sister had a fancy gold fish in a bowl. I never saw it swim around, it just sat there. I assume it was due to the cramped space it was living in. It did live for a couple years, so it wasn't sick or anything.