"Do I have to still report that $300 gain in my 2020 income, even though it would be reported in 2021 1099-R?"
Yes and yes.
Do you know exactally how it will b e reported? (A Roth or Traditional IRA return and what code the IRA custodian will use in box 7 on the 1099-R)?
This *can* be reported now with a "dummy" 1099-R but only if you *know* how it will be reported. Roth and Traditional IRA's returned contributions are reported with different codes.