IPSec tunnel, gateway routes, create gateway

[klmbees]

I have successfully established an IPSec tunnel between a home office site and a branch site. The branch site firewall was recently replaced by a pfsense whitebox as the existing firewall was no longer meeting the demands of the business.

The tunnel is established as follows:
SITEA = pfsense - BRANCH OFFICE
SITEB = (other guys firewall) - HEADQUARTERS

tunnels are ESTABLISHED routing some local subnets. Currently, SITEA internet traffic egresses through SITEA WAN, and Site B does the same.

I need traffic from ONLY CERTAIN subnets/vlans within SITEA (but not its entire summary) to egress VIA THE IPSEC TUNNEL and to SITEB for internet-bound traffic.

As I understand it, I can do this via advanced firewall rule "change gateway to" - but I cannot figure out how to make a gateway that exists at SITE B on the pfsense. I tried routing 0.0.0.0/0 over the ipsec tunnel, broke routing on the pfsense at SITEA.

any help is appreciated.