The following billing model applies to all vCPUs, GPUs, and memory resources. The billing model also applies to several premium images that you run on Compute Engine instances.
All vCPUs, GPUs, and GB of memory are charged a minimum of 1 minute. For example, if you run your virtual machine for 30 seconds, you will be billed for 1 minute of usage.
After 1 minute, instances are charged in 1 second increments.
For more info refer https://cloud.google.com/compute/vm-instance-pricing