Let's say that I got 0.01 BTC for $500 in 2021 and I'm sure that I sold it later during the year, but don't know when or for how much. How do I calculate the gain?
If you don't know how much you sold it for, obviously, you won't be able to calculate it. I understand that you bought it in a CEX and probably sold it in a CEX as well? If so there will be a history at the CEX. Because if it was in a decentralized exchange I don't think you would be eating your head off to declare it, let alone such a small amount. How much would be the profit? $200? $500?
Give us more details, otherwise we will have to consult our crystall ball, as NotFuzzyWarm points out.