On their website they announce a free machine:
So, how can I get this free machine?
You've got to take a leap of faith.
There are two kinds of Free tiers:
The Google Cloud Platform Free Tier is your opportunity to learn and use GCP for free. It has two parts: a 12-month, $300 credit free trial and Always Free. The 12-month, $300 free trial allows you to use any GCP product. Always Free allows you to try participating products for free up to their non-expiring usage limits, making it easy for you to test and develop with these products.
Regarding the "Always Free" part, until the very end it's going to show you a price, but if you keep below the Free Tier specs you won't be billed.
Just to be sure I contacted their suport and was answered (bold added by me):
I understand that you want to use a free f1-micro instance type but bothered to use it due to showing estimated monthly price of $4.28.
As you are eligible for ¨Always Free¨, 1 f1-micro is free of charges per month as you can see on this link . In case you get charges using 1 f1-micro please contact our billing support using this link