A GCP VM behaves pretty much like any other computer System, networking wise; in the case that two network interfaces are present, it will handle the traffic according to its routing table. Most probably, your system has automatically configured a default gateway in an attempt to reach any non-directly connected network through the ens4 interface, since it was the first one added to the VM, and therefore all the traffic is going through it.
If you want to change this behavior, you would need to statically configure the routing table of your system or to use some dynamic routing protocol.
That being said, a system that has several network interfaces is usually one that is intended to be used as a network appliance for services as Load Balancing, Intrusion Detection and Protection, Web Application Firewall, etc. These services usually run high resource consumption processes and it is not recommended to use just any VM for that. Depending on what you want to achieve by having two interfaces in a VM (high availability, load balancing, etc.) might be better solutions to explore.
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