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?
He can still keep track of his trading history if he some through CEX or DEX.
All he has to do is to check his wallet history for the year the trading was executed if sold through DEX since he knows the quantity of the BTC.