As discussed earlier, it is possible to create a custom subnet in an already existing VPC. Follow these steps:

1. Navigate to the VPC networking page and choose one of the available networks.
2. Click on the Add a subnet option:

3. Fill in the details for name, region, IP range, and secondary IP range. Make sure you don't overlap IP ranges with any other existing subnet in your VPC.
4. After deciding to opt for logs and Private Google Access, click on the Add option.
5. Alternatively, you can use following command in the cloud shell:

    gcloud compute networks subnets create <<<SUBNET_NAME>>> 
    --network <<<NETWORK>>> 
    --range <<<IP_RANGE>>> 
    [--secondary-range <<<RANGE_NAME>>>=<<<2ND_IP_RANGE>>>

6. To describe any subnet, use the following command:

    gcloud...