By default when you create a Compute Engine virtual machine in Google Cloud, you get public IP address. This IP address changes when you ever shutdown down your instance.
To avoid IP getting changed, you need to reserve the IP address to do this, you need to change type of IP address from Ephemeral to Static.
Google Cloud Platform > Networking > VPC Network > External IP addresses
You will see list of all IP associated with your instances here.
Under Type column, set type to Static. You will be asked to enter a name and description. Once you do this, IP will not release even if you shutdown a VM.