Connecting to Compute Instance running in the private subnet of GCP VPC from your DevBox.
Follow Connecting to GCP guide.
Go to Compute Engine > VM Instances.
Click on SSH. You will see the commands for accessing it.
Go to DevBox and make a copy of the key pair.
Connect to Compute Instance:
Go to Compute Engine > Create Instance.
Enter the name of the instance along with region and machine type.
Use one of the supported linux distros.
Choose the Instance type.
Go to Advanced Options > Networking > Network Tags and enter the firewall tags.
Go to Advanced Options > Networking > Network Interfaces, click on Network and choose the VPC where your bastion host is running, then click on Subnetwork and choose the private subnet.
Copy the newly created SSH key pair once the instance is created.
Go to the DevBox and make a copy of the key pair.
Connect to the desired Compute instance using ssh and the new key pair: