Virtual Server using external site as pool member, routing problem?

I believe my issue is pretty straight forward but I'm not able to get it going. I'm not able to use an external host ( a host that my F5 does not have direct access to via one of its vlans ) as a pool number. No issues when using any hosts on any of the different vlans connected directly to the f5. I imagine the virtual host doesn't know how to route out of the network to access the external host, I'm just not sure where exactly I should be defining a "Default Gateway" for the f5 to route things?