in your case there is no point in assigning a gateway for the lan interface afaik .
all you'd have to do is add static routes (for your various lan subnets) pointing towards your core-switch-router-thing, that sits internally on your lan and handles the routes.
on your core-switch-routing-thing, you'd need a route (for your openvpn-tunnel-network) pointing to pfsense.
Offcourse you could also choose not todo this all manually and use a dynamic routing protocol, like ospf,rip, … to add the routes for you.