GTM to return NAT IPs without LTM

Welcome! When you say DC1-NAT-IP , what is this IP on? Is it a NAT IP on a firewall? You then mention that you can't do any checking because it is a NAT IP. Why can't you check a NAT IP? Won't the check of the NAT IP just pass through to the backend server that you care to monitor?

One thing to note is that GTM will only hand out an IP address of an object that is has marked as Green (available), or a static IP like you were talking about having for Fallback IP.

You could create a pool using the global availability load balancing method. The problem is that you'd need to create some sort of monitor that would ALWAYS be up until you manually disabled it.

pool-blah - DC1-NAT - DC2-NAT

I did something like that one time where I created a monitor that pinged the upstream router, figuring if that's down, I'm dead in the water anyway.

The upstream router was very reliable, so all I had to do was create that monitor, assign it to the pool-blahand then I just had to disable DC1-NAT whenever I wanted to shift traffic.

Honestly, using the dummy pool with fallback IP would accomplish the same thing with less complexity probably.