Unfortunately, we recently have closed a ticket about this, and the solution is:
Quantum Spark Appliances in Centrally Managed mode DO NOT fully support the configuration with Single Routable IP and interfaces on different network even if it's confirmed by documentation (https://sc1.checkpoint.com/documents/SMB_R81.10.X/AdminGuides_Centrally_Managed/EN/Content/Topics/Co...), this seems to be available only for Quantum Force and higher (https://sc1.checkpoint.com/documents/R81.10/WebAdminGuides/EN/CP_R81.10_ClusterXL_AdminGuide/Topics-...).
You can try to configure a new "local transport network" between gateway and router so you can have as many IP as you want to configure in each interface. In this case you cannot access directly each cluster member from internet, but you can do so via DNAT.
Something like that:
Router External: 1.1.1.1
Router Internal: 192.168.1.1/24
Checkpoint External: 192.168.1.2/24
Checkpoint External gateway: 192.168.1.1
Hoping to be useful