VPC peering
VPC peering allows traffic to be freely routed between two VPCs. Once peered, the two VPCs act as one with respect to connectivity. However, network security group rules are still observed. VPC peering is simple and easy to set up, but the limitation is that IP ranges of two peered VPCs can't overlap.
For an example of VPC peering see Connect to BigAnimal private cluster using GCP VPC peering for step-by-step instructions in the knowledgebase.