Introducing QUIC support for HTTPS load balancing explains how QUIC is supported for a client <-> load balancer connection. My upstream server uses aioquic and accepts only HTTP/3 connections attempted proactively, without support for upgrading from HTTP/1.1 or HTTP2. Can I configure Cloud Load Balancer to always communicate with my upstream servers in Google Kubernetes Engine via HTTP/3?
As of now you cannot use QUIC/HTTP3 backend, when using the HTTP(S) loadbalancing the following backend protocols are supported:
HTTP, HTTPS, HTTP/2
Please see this for more info on backend protocols supported.