External IP addresses attached to free vms are also free. From network pricing page
Note: Starting January 1st, 2020, GCP will introduce an additional charge for publicly addressed VM instances that don’t fall under the Free Tier. You will not be charged for other publicly addressed resources, such as forwarding rules.
and from the free tier page
Your Always Free
f1-micro instance limit is by time, not by instance. Each month, eligible use of all of your
f1-micro instances and associated external IP addresses are free until you have used a number of hours equal to the total hours in the current month