Update: I’ve found the issue.

server {
    listen 12345 ssl http2;
    server_name grpcgateway;
    default_type application/grpc;

    ssl_session_cache builtin:1000 shared:SSL:10m;
    ssl_session_timeout  5m;

    location / {
       grpc_set_header X-Forwarded-For $remote_addr;
       grpc_pass grpc://abc-server:12345;
    }
}

The grpc protocol here should be changed to grpcs:

grpc_pass grpcs://abc-server:12345;