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.
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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.
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After 1 minute, instances are charged in 1 second increments.
For more info refer https://cloud.google.com/compute/vm-instance-pricing