When none of the CLI tools like istioctl or glooctl are available, use wget/curl | jq/grep to find all the hostnames configured in Envoy
Assuming Envoy admin port is 19000. Invoke below script to invoke Envoy admin API
$ cat collect-gloo-upstream.sh
#!/usr/bin/env sh
kubectl -n $1 exec -it $2 -- wget -O - http://127.0.0.1:19000/clusters?format=json