Is it a single ISP or the same ISP with two different NAT numbers?
If they are different ISPs, you should be able to accomplish this via ISP Redundancy.
See: https://support.checkpoint.com/results/sk/sk34812
Otherwise, you should use a Dynamic Object in your NAT rule instead (which is what ISP Redundancy ultimately does).
You'll need to write a script to update the contents of this Dynamic Object using the dynamic_objects CLI command on each gateway that uses this object.
However, this gives you flexibility as to how and when to "fail over" the NAT.