Hello!
First of all, you'd have to use route-based VPN as you said, instead of a pure policy-based VPN.
So what you do is define two VTI interfaces on the gateway, acting as the logical interfaces for the VPN, and then set up routing based off that, where you also set up which third party gateway it will communicate towards.
If you want to use static routes with IP tracking, or dynamic protocols such as OSPF or BGP is up to you, I would personally recommend dynamic protocol.
See this guide when it comes to the VTIs etc:
https://sc1.checkpoint.com/documents/R81/WebAdminGuides/EN/CP_R81_SitetoSiteVPN_AdminGuide/Topics-VP...
As for the VPN itself, you create a policy based VPN as usual, but leave the VPN domains as empty groups, (since the routing will decide what will traverse over the tunnel).