Edgegateway required one external and one internal Network. And we can create 3 types of orgVDCNetworks. 1. Isolated neworks, 2. NAT Routed Networks. 3. Bridged networks. Edgegateway is needed when we are creating NAT Routed networks.
you can further read the details from following API referencence.
http://pubs.vmware.com/vcd-51/topic/com.vmware.ICbase/PDF/vcd_51_api_guide.pdf
Page 148, Topic: About vCloud Director Networks
Page 151: Topic: Create an Edge Gateway
I hope these topics explains well about your question.