flush the old gateway by going into
smitty route
--> select the last option
--> flush all the gateways from ODM ( last two options SET TO YES / YES )

After flushing ... exit...check if the gateway is there or not by #netstat -Crn
not present...good...then add your new gateway.

# route add default IP_ADDRESS

to make it permanent go to smitty route
add static route