If your gateway is deployed in the same Amazon account as your servers,
you might want to look at a VPC solution where you can control the instances'
private IPs using masks.
If that is not an option, you will need to write a script, which you
should add to the Launch Configuration's User Data.
In this script you can use AWS CLI to find which IP Addresses are available using
describe-addresses, and use one of them to
associate to your newly created instance using