Have you considered buying a Pi Zero W and running Pi-Hole locally? That's a startup cost of about $15 and after that your only cost is a dollar or so of electricity per year, if you pay for your electricity separately.
You can set up a VPN back to your home Pi and use that while traveling. You can split tunnel if you wish, and route only the DNS traffic home while the data traffic runs on whichever network the client is on.
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.
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