This USG configuration Redirects DNS traffic to my pihole at .9, .10 and .11. In addition forwarded traffic is disguised so the client will not recognize that he did not hit 8.8.8.8

config.gateway.json

{
   "service": {
      "nat": {
         "rule": {
            "1": {
               "description": "DNS Redirect to pihole",
               "destination": {
                  "address": "!192.168.1.1-192.168.1.11",
                  "port": "53"
               },
               "source" : {
                  "address": "!192.168.1.1-192.168.1.11"
               },
               "inbound-interface": "eth1",
               "inside-address": {
                  "address": "192.168.1.9-192.168.1.11",
                  "port": "53"
               },
               "log": "disable",
               "protocol": "tcp_udp",
               "type": "destination"
            }
         }
      }
   }
}