I've to admit that I don't understand your point here
Perhaps because you don't tell enough about your configuration or because you do not tell what's the error message when trying to do it using Zentyal GUI
I'm not using 3.x but I believe that except for fail-over and load balancing mechanisms that are provided with Zentyal, GUI will not allow to configure, using "static routes" options, different routes for same destination. This is pretty obvious at least to me (or there is something I missed here)
Furthermore, it looks like, if I understand well, that your target gateways for this rule are your ISP, so I suppose this should be handled by your default "fail-over" or "load balancing" configuration.
As you keep asking, I suppose this is not but you should explain why so that someone could help further
my $0.02