You must add the route from the Gaia clish shell or from the Gaia web interface. Assuming it is valid, the route will take effect immediately and a policy install is not required.
There was an R77.30 feature called Central Device Management that could be enabled in the Global Properties that would let you manipulate static routes right on the firewall object in SmartDashboard, but this feature was not integrated directly into the R80+ SmartConsole and seems to only be part of the separate SmartProvisioning GUI now.
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