I don't know how the gambling site works in converting BTC to FIAT through win or lose,my assuming gambling sites usually already have BTC and win or lose already in BTC units.If changing to FIAT is related to trade but if I read your title about US tax,it seems that it was correct right I have time to read some sites related about it.
If we win on gambling sites and keeping them as bitcoin will make us no worry to pay taxes since it is not traceable but when we are trading them or converting them through exchanges you will get recorded by exchange's contract address so you will get caught if you are cheating to pay the taxes to governments.