If I recall correctly you have to create a link network between them that you add to your VPC.
As far as I know you cannot have the same ip network on both sides of the gateway.
Cloud network inside the VPC: 10.17.20.0/24
External customer network: 10.17.21.0/24
Link network: 10.17.22.0/30, vlan 41 or whatever you have available
Inside the link network you give your VPC the ip address 10.17.22.2 and in the gateway you put 10.17.22.1.
On 10.17.22.1 you configure a router to route between the networks.