Billing during the free trial
When you begin the free trial, Google creates a billing account for you and credits $300 to your account. Anything you do that would normally result in a charge is billed against this credit. You will receive billing statements indicating how much of your free credit has been spent.
Note: You should be aware that if you do not upgrade to a paid account after the free trial ends, your Google Compute Engine resources might be removed. Removed resources are not recoverable.