Free as in you pay for your worker nodes, but nothing additional for operating the platform itself. You do lose control over your control plane nodes and etcd however.
If you go to the GCP pricing calculator[0] and add one n1-standard-1 VM to GCE, it costs $24.27/month.
Add one n1-standard-1 VM to a GKE cluster and you pay $38.27/month. There is a literal line item saying "Kubernetes Engine Cost: USD 14.00".
It's not free. It's just billed under the worker.
Edit: ah, I see my mistake. There's a dropdown that lists "Advanced", which apparently accounts for the $14 difference.