Update loadgen.sh (#15047)
Update loadgen.sh (#15047)

change HELLOWORLD_URL to GATEWAY_URL
geeknoid/istio,smawson/istio,istio/istio,geeknoid/istio,geeknoid/istio,geeknoid/istio,smawson/istio,istio/istio,geeknoid/istio,smawson/istio,istio/istio,istio/istio,geeknoid/istio,istio/istio,smawson/istio,smawson/istio,smawson/istio