We need this to use non standard TCP port. From what I can see Load balancers need instance groups to be defined, and instance groups are compute engine thing. So I wonder if it's possible at all.
App Engine is a managed service, this means that it has it's own load balancer and manages the requests in a very specific way which you can check over here.
As mentioned by @guillaume blaquiere, you would be mixing 2 concepts completely different which are IaaS and PaaS. If you need to set up a specific load balancer with certain behaviour for your App to run, you may be better off using Managed instance groups rather than App Engine so you have more control on everything rather than relying on GCP's management.
I hope you find this information useful.