So, i’m not sure this is the best way to do it, but i have successfully done something like that using the Balancing Rules found under Net Balancing.
There you can
* create your two gateways
* set them as Load Balancing and Failover
* give them identical weight
* force traffic through one or the other using the Balancing Rules
In the Balancing Rules you can filter to whatever you want, and send to whatever Target Gateway you wish