The following is a guide for adding a Floating IP that will be used on the NAT Gateway:
On the NAT Gateway Configuration page, select the Floating IP label then click Create Floating IP.
Add Floating IP
Select the network used and click Create to continue the process.
Add Floating IP
The notification "Floating IP [floating IP name] has been successfully created" appears that the Floating IP has been successfully added to the NAT Gateway.