You will have to know the gateway you want to push.
What routes do you want to push thru desired gateway? In generak, use:
--route network/IP [netmask] [gateway] [metric]
in your OpenVPN on GW1.
For example, to redirect all trafic use:
push "route 0.0.0.0 128.0.0.0 GW2_IP"
push "route 128.0.0.0 128.0.0.0 GW2_IP"
