How to mitigate 12 AWG to 14 AWG transition at GFCI
You should change the breaker to 15A, or replace the wire. It is not OK to have 14 Ga wire on a 20A breaker like that.
The GFCI is not an over current protection device and does not help you here. It will happily let 20A through and light the 14 Ga wire on fire as long as there’s no imbalance between hot and neutral.