Configure Policy-Based Routing (PBR) specifying a different default IP next hop for interested traffic by issuing a route-map command. This is a sample configuration for the traffic sourced from 10.1.1.0/24, coming in on interface Ethernet0, policy routed to next-hop x.x.x.x:
ip address 10.1.1.1 255.255.255.0
ip policy-map new-gateway
route-map new-gateway permit 10
match ip address 10
set ip default next-hop x.x.x.x
access-list 10 permit 10.1.1.0 0.0.0.255
Note: The traffic that does not match the policy uses the default route configured in the router.