Problems pointing traffic through GRE tunnel

[uuuji]

I recently have GRE over IPSec and ospf settled up that I can connect my machines on the remote site fine, however I also want to redirect some of my subnets' traffic through the tunnel so network traffics from those subnets will appear to be coming from my remote IP.

I tried to set an outbound rule that NAT the traffic to my GRE gateway, which nothing happened and traffics from those subnets still appear that it comes from my host IP; I also tried to change those subnets' gateway to GRE tunnel, but that way all traffic from those subnets will stuck at router(traceroute only shows router IP) and clients in the subnet can't even access other machines in the local network. Given that I don't want all subnets' traffic to go through GRE tunnel so I didn't try to add static routes, and I'm still testing this box so currently my firewall rules is just allowing all traffic to pass.

I'm kinda out of ideas to solve this now and curious if I can get any help/advice on this.

Thank you