Google Global HTTP(S) Load Balancing only forwards traffic on TCP 80, 8080, and 443 by definition.
This limitation is due to the internal architecture of the load balancing system and the way it forwards request to its backends.
Global HTTP(S) Load Balancers are proxies. They terminate HTTP or HTTPS connections using a server that's not in your GCP project, then send requests to the backend instances for which they have been configured. link1
When you create a Global HTTPS Load Balancer, including its public IP address, other front-end components (forwarding rule and target proxy), URL map, and backend service(s), you're creating a set of programming information that applies to a fleet of Google Front End (GFE) servers worldwide. Each GFE server is the proxy, and it's not part of your instance link2.
Did you have a look into setting up a SSL proxy load balancer ? See this link link3