What kind of gold fish you have? Common or Fancy?
The conservative answer is: other goldfish .... and you generally should not mix common & fancies.
You can however consider these other cold water fish if you have fancies <some members will have a fit when they see golds with others, but it is possible, esp in large tanks>:
Hill stream Loaches (aka Hong Kong or Butterfly Pleco) - need high current & high O2 content.
Dwarf Pleco - ie rubberlip or bristlenose - warning, other plecos have been known to attack golds, and very small plecos may be eaten by the gold & get stuck in the gold's throat ... if you go the dwarf pleco route, make sure you get a largish one.
White cloud mountain minnows - safest bet for the golds. Golds will eat the small ones, so again, you need to get bigger specimens, and a largish school ... & make sure you have plenty of plants & other hiding space. The gold will eat the occasional minnow, but with a fair size school, they should reproduce enough to keep up with the losses.
Corys - some have had success with cold water species - but others have found large golds trying to eat the corys & since corys are barbed & armoured, will get lodge in the golds throat & kill it. IMO - this is a bit risky.
Snails - not a fish, I know, but interesting creatures & fine with golds.
Some people have mix golds with true tropical, THAT is generally not recommended.