GCP is charging fees for GKE Cluster Management fees
Spoke with GCP support. Yes its free every month even after free trail ends but they only start giving $74 free credit only after you upgrade your account to paid account.
As for the existing $300 free credit, kubernetes Engine charges will get deducted and in the report it'll show under discount (which is for paid account).
So technically $300 free credits include kubernetes engine charges but paid account users get $74 free credit every month for K8s cluster.